Allow Symbian widget implementations to select native paint mode

On the Symbian platform, the Qt raster paint engine targets an
off-screen buffer owned by the Font & Bitmap server (FBSERV).
When an area of the screen needs to be refreshed, the window
server (WSERV) asks the control environment (CONE) to redraw the
control(s) intersecting that screen region.  Each Qt native
widget has an associated Symbian control, whose Draw function
blits the required region of the backing store via WSERV.

Use cases involving Direct Screen Access (DSA) may require this
behaviour to be modified, to either of the following:

    - Disable: the Draw function does nothing.  In this case,
    the output of paint events, rendered to the backing store,
    is not blitted to the screen.  This mode was introduced by
    change 8f445e13.

    - Zero fill: the Draw function fills all pixels within the
    redraw region with zeroes.

This change allows the widget implementation to select either of
these alternative modes by setting a flag in its QWExtra structure.

Note that these alternative modes are only suitable for native
widgets, because they act on a per-control rather than per-widget
basis.

Task-number: QTBUG-5467
Reviewed-by: Jason Barron

3 files changed, 50 insertions, 8 deletions

diff --git a/src/gui/kernel/qapplication_s60.cpp b/src/gui/kernel/qapplication_s60.cpp
index fb2bc72..ae7b494 100644
--- a/src/gui/kernel/qapplication_s60.cpp
+++ b/src/gui/kernel/qapplication_s60.cpp
@@ -822,10 +822,31 @@ void QSymbianControl::Draw(const TRect& controlRect) const
         CFbsBitmap *bitmap = s60Surface->symbianBitmap();
         CWindowGc &gc = SystemGc();
 
-        if(!qwidget->d_func()->extraData()->disableBlit) {
+        switch(qwidget->d_func()->extraData()->nativePaintMode) {
+        case QWExtra::Disable:
+            // Do nothing
+            break;
+
+        case QWExtra::Blit:
             if (qwidget->d_func()->isOpaque)
                 gc.SetDrawMode(CGraphicsContext::EDrawModeWriteAlpha);
             gc.BitBlt(controlRect.iTl, bitmap, backingStoreRect);
+            break;
+
+        case QWExtra::ZeroFill:
+            if (Window().DisplayMode() == EColor16MA) {
+                gc.SetBrushStyle(CGraphicsContext::ESolidBrush);
+                gc.SetDrawMode(CGraphicsContext::EDrawModeWriteAlpha);
+                gc.SetBrushColor(TRgb::Color16MA(0));
+                gc.Clear(controlRect);
+            } else {
+                gc.SetBrushColor(TRgb(0x000000));
+                gc.Clear(controlRect);
+            };
+            break;
+
+        default:
+            Q_ASSERT(false);
         }
     } else {
         surface->flush(qwidget, QRegion(qt_TRect2QRect(backingStoreRect)), QPoint());
diff --git a/src/gui/kernel/qwidget_p.h b/src/gui/kernel/qwidget_p.h
index 66efcb5..025d703 100644
--- a/src/gui/kernel/qwidget_p.h
+++ b/src/gui/kernel/qwidget_p.h
@@ -230,12 +230,33 @@ struct QWExtra {
 #elif defined(Q_OS_SYMBIAN) // <----------------------------------------------------- Symbian
     uint activated : 1; // RWindowBase::Activated has been called
 
-    // If set, QSymbianControl::Draw does not blit this widget
-    // This is to allow, for use cases such as video, widgets which, from the Qt point
-    // of view, are just placeholders in the scene.  For these widgets, any necessary
-    // drawing to the UI framebuffer is done by the relevant Symbian subsystem.  For
-    // video rendering, this would be an MMF controller, or MDF post-processor.
-    uint disableBlit : 1;
+    /**
+     * Defines the behaviour of QSymbianControl::Draw.
+     */
+    enum NativePaintMode {
+        /**
+         * Normal drawing mode: blits the required region of the backing store
+         * via WSERV.
+         */
+        Blit,
+
+        /**
+         * Disable drawing for this widget.
+         */
+        Disable,
+
+        /**
+         * Paint zeros into the WSERV framebuffer, using BitGDI APIs.  For windows
+         * with an EColor16MU display mode, zero is written only into the R, G and B
+         * channels of the pixel.
+         */
+        ZeroFill,
+
+        Default = Blit
+    };
+
+    NativePaintMode nativePaintMode : 2;
+
 #endif
 };
 
diff --git a/src/gui/kernel/qwidget_s60.cpp b/src/gui/kernel/qwidget_s60.cpp
index 359df2a..d7d76df 100644
--- a/src/gui/kernel/qwidget_s60.cpp
+++ b/src/gui/kernel/qwidget_s60.cpp
@@ -881,7 +881,7 @@ void QWidgetPrivate::deleteTLSysExtra()
 void QWidgetPrivate::createSysExtra()
 {
     extra->activated = 0;
-    extra->disableBlit = 0;
+    extra->nativePaintMode = QWExtra::Default;
 }
 
 void QWidgetPrivate::deleteSysExtra()