Workaround to VideoCore III scissor bug.

Some versions of VideoCore III drivers seem
to pollute and use stencil buffer when using
glScissors. Workaround is to clear stencil buffer
before disabling scissoring.

Task-number: QT-5308
Reviewed-by: Samuel Rødal

Author: Jani Hautakangas

src/opengl/gl2paintengineex/qpaintengineex_opengl2.cpp

@@ -2059,6 +2059,8 @@ void QGL2PaintEngineExPrivate::updateClipScissorTest()
     currentScissorBounds = bounds;
 
     if (bounds == QRect(0, 0, width, height)) {
+        if (ctx->d_func()->workaround_brokenScissor)
+            clearClip(0);
         glDisable(GL_SCISSOR_TEST);
     } else {
         glEnable(GL_SCISSOR_TEST);

src/opengl/qgl.cpp

@@ -1716,6 +1716,8 @@ void QGLContextPrivate::init(QPaintDevice *dev, const QGLFormat &format)
     workaround_brokenTextureFromPixmap = false;
     workaround_brokenTextureFromPixmap_init = false;
 
+    workaround_brokenScissor = false;
+
     for (int i = 0; i < QT_GL_VERTEX_ARRAY_TRACKED_COUNT; ++i)
         vertexAttributeArraysEnabledState[i] = false;
 }

src/opengl/qgl_egl.cpp

@@ -192,7 +192,9 @@ void QGLContext::makeCurrent()
         if (!d->workaroundsCached) {
             d->workaroundsCached = true;
             const char *renderer = reinterpret_cast<const char *>(glGetString(GL_RENDERER));
-            if (renderer && (strstr(renderer, "SGX") || strstr(renderer, "MBX"))) {
+            if (!renderer)
+                return;
+            if ((strstr(renderer, "SGX") || strstr(renderer, "MBX"))) {
                 // PowerVR MBX/SGX chips needs to clear all buffers when starting to render
                 // a new frame, otherwise there will be a performance penalty to pay for
                 // each frame.
@@ -229,6 +231,13 @@ void QGLContext::makeCurrent()
                         d->workaround_brokenFBOReadBack = true;
                     }
                 }
+            } else if (strstr(renderer, "VideoCore III")) {
+                // Some versions of VideoCore III drivers seem to pollute and use
+                // stencil buffer when using glScissors even if stencil test is disabled.
+                // Workaround is to clear stencil buffer before disabling scissoring.
+
+                // qDebug() << "Found VideoCore III driver, enabling brokenDisableScissorTest"; 
+                d->workaround_brokenScissor = true;
             }
         }
     }

src/opengl/qgl_p.h

@@ -415,6 +415,8 @@ class QGLContextPrivate
     uint workaround_brokenTextureFromPixmap : 1;
     uint workaround_brokenTextureFromPixmap_init : 1;
 
+    uint workaround_brokenScissor : 1;
+
     QPaintDevice *paintDevice;
     QColor transpColor;
     QGLContext *q_ptr;