Added a public function to enforce usage og the old GL engine.

Some applications that uses a mix of OpenGL and QPainter code may
not work correctly with the new GL 2 engine (e.g. the composition demo).
The same is most likely also true for user apps, therefore we need
a way to enforce the usage of the old GL 1 engine for the sake of
compatibility.

Task-number: 260872
Reviewed-by: Samuel

5 files changed, 87 insertions, 11 deletions

diff --git a/demos/composition/main.cpp b/demos/composition/main.cpp
index 3a959a9..fe142ad 100644
--- a/demos/composition/main.cpp
+++ b/demos/composition/main.cpp
@@ -42,11 +42,17 @@
 #include "composition.h"
 
 #include <QApplication>
+#ifdef QT_OPENGL_SUPPORT
+#include <QtOpenGL>
+#endif
 
 int main(int argc, char **argv)
 {
  //   Q_INIT_RESOURCE(deform);
 
+#ifdef QT_OPENGL_SUPPORT
+    QGL::setPreferredPaintEngine(QPaintEngine::OpenGL);
+#endif
     QApplication app(argc, argv);
 
     CompositionWidget compWidget(0);
diff --git a/dist/changes-4.6.0 b/dist/changes-4.6.0
index ca984ac..f9984d3 100644
--- a/dist/changes-4.6.0
+++ b/dist/changes-4.6.0
@@ -54,8 +54,15 @@ information about a particular change.
    this is that Nokia focuses on OpenGL for desktop hardware accelerated
    rendering.
 
- - When mixing OpenGL and QPainter calls you need to surround your custom
-   OpenGL calls with QPainter::beginNativePainting() and
+ - The default engine used to draw onto OpenGL buffers has changed in
+   Qt 4.6. The QPaintEngine::OpenGL2 engine is now used as the default
+   engine. This *may* cause compatibility problems for applications
+   that use a mix of QPainter and native OpenGL calls to draw into a GL
+   buffer. Use the QGL::setPreferredPaintEngine() function to enforce
+   usage of the old GL paint engine.
+
+ - When mixing OpenGL and QPainter calls you need to surround your
+   custom OpenGL calls with QPainter::beginNativePainting() and
    QPainter::endNativePainting().
    This is to ensure that the paint engine flushes any pending drawing and sets
    up the GL modelview/projection matrices properly before you can issue custom
diff --git a/src/opengl/qgl.cpp b/src/opengl/qgl.cpp
index 087902b..02bb8f9 100644
--- a/src/opengl/qgl.cpp
+++ b/src/opengl/qgl.cpp
@@ -141,6 +141,48 @@ QGLSignalProxy *QGLSignalProxy::instance()
     return theSignalProxy();
 }
 
+
+class QGLEngineSelector
+{
+public:
+    QGLEngineSelector() : engineType(QPaintEngine::MaxUser) { }
+
+    void setPreferredPaintEngine(QPaintEngine::Type type) {
+        if (type == QPaintEngine::OpenGL || type == QPaintEngine::OpenGL2)
+            engineType = type;
+    }
+
+    QPaintEngine::Type preferredPaintEngine() {
+        if (engineType == QPaintEngine::MaxUser) {
+            // No user-set engine - use the defaults
+#if defined(QT_OPENGL_ES_2)
+            engineType = QPaintEngine::OpenGL2;
+#else
+            // We can't do this in the constructor for this object because it
+            // needs to be called *before* the QApplication constructor
+            if ((QGLFormat::openGLVersionFlags() & QGLFormat::OpenGL_Version_2_0)
+                && qgetenv("QT_GL_USE_OPENGL1ENGINE").isEmpty())
+                engineType = QPaintEngine::OpenGL2;
+            else
+                engineType = QPaintEngine::OpenGL;
+#endif
+        }
+        return engineType;
+    }
+
+private:
+    QPaintEngine::Type engineType;
+};
+
+Q_GLOBAL_STATIC(QGLEngineSelector, qgl_engine_selector)
+
+
+bool qt_gl_preferGL2Engine()
+{
+    return qgl_engine_selector()->preferredPaintEngine() == QPaintEngine::OpenGL2;
+}
+
+
 /*!
     \namespace QGL
     \inmodule QtOpenGL
@@ -181,6 +223,32 @@ QGLSignalProxy *QGLSignalProxy::instance()
     \sa {Sample Buffers Example}
 */
 
+/*!
+   \fn void QGL::setPreferredPaintEngine(QPaintEngine::Type engineType)
+
+   \since 4.6
+
+   Sets the preferred OpenGL paint engine that is used to draw onto
+   QGLWidgets, QGLPixelBuffers and QGLFrameBufferObjects with QPainter
+   in Qt.
+
+   The \a engineType parameter specifies which of the GL engines to
+   use. Only \c QPaintEngine::OpenGL and \c QPaintEngine::OpenGL2 are
+   valid parameters to this function. All other values are ignored.
+
+   By default, the \c QPaintEngine::OpenGL2 engine is used if GL/GLES
+   version 2.0 is available, otherwise \c QPaintEngine::OpenGL is
+   used.
+
+   \warning This function must be called before the QApplication
+   constructor is called.
+*/
+void QGL::setPreferredPaintEngine(QPaintEngine::Type engineType)
+{
+    qgl_engine_selector()->setPreferredPaintEngine(engineType);
+}
+
+
 /*****************************************************************************
   QGLFormat implementation
  *****************************************************************************/
diff --git a/src/opengl/qgl.h b/src/opengl/qgl.h
index daac760..b110665 100644
--- a/src/opengl/qgl.h
+++ b/src/opengl/qgl.h
@@ -43,6 +43,7 @@
 #define QGL_H
 
 #include <QtGui/qwidget.h>
+#include <QtGui/qpaintengine.h>
 #include <QtOpenGL/qglcolormap.h>
 #include <QtCore/qmap.h>
 #include <QtCore/qscopedpointer.h>
@@ -130,6 +131,8 @@ class QGLContextPrivate;
 // Namespace class:
 namespace QGL
 {
+    Q_OPENGL_EXPORT void setPreferredPaintEngine(QPaintEngine::Type engineType);
+
     enum FormatOption {
         DoubleBuffer            = 0x0001,
         DepthBuffer             = 0x0002,
diff --git a/src/opengl/qgl_p.h b/src/opengl/qgl_p.h
index d4b7597..f8158a0 100644
--- a/src/opengl/qgl_p.h
+++ b/src/opengl/qgl_p.h
@@ -519,15 +519,7 @@ extern QPaintEngine* qt_qgl_paint_engine();
 extern EGLDisplay qt_qgl_egl_display();
 #endif
 
-inline bool qt_gl_preferGL2Engine()
-{
-#if defined(QT_OPENGL_ES_2)
-    return true;
-#else
-    return (QGLFormat::openGLVersionFlags() & QGLFormat::OpenGL_Version_2_0)
-           && qgetenv("QT_GL_USE_OPENGL1ENGINE").isEmpty();
-#endif
-}
+bool qt_gl_preferGL2Engine();
 
 inline GLenum qt_gl_preferredTextureFormat()
 {