Cut performance cost in QSGContext::prepareMaterial()

The hash-lookup in this function is costing us a lot, and
since we're looking up the same materials again and again,
and the material has a place holder for it, we can store it
directly in the material at no extra cost. This was a 10%
gain in this particular benchmark.

Change-Id: I46b67791ce39f453fa86d1ee82f6f5c7785b46a1
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/quick/scenegraph/coreapi/qsgmaterial.cpp

@@ -556,6 +556,7 @@ static void qt_print_material_count()
 
 QSGMaterial::QSGMaterial()
     : m_flags(0)
+    , m_reserved(0)
 {
 #ifndef QT_NO_DEBUG
     if (qsg_leak_check) {

src/quick/scenegraph/coreapi/qsgmaterial.h

@@ -133,6 +133,7 @@ public:
     void setFlag(Flags flags, bool on = true);
 
 private:
+    friend class QSGContext;
     Flags m_flags;
     void *m_reserved;
     Q_DISABLE_COPY(QSGMaterial)

src/quick/scenegraph/qsgcontext.cpp

@@ -478,10 +478,16 @@ QSGDepthStencilBufferManager *QSGContext::depthStencilBufferManager()
 QSGMaterialShader *QSGContext::prepareMaterial(QSGMaterial *material)
 {
     Q_D(QSGContext);
+
+    if (material->m_reserved)
+        return reinterpret_cast<QSGMaterialShader *>(material->m_reserved);
+
     QSGMaterialType *type = material->type();
     QSGMaterialShader *shader = d->materials.value(type);
-    if (shader)
+    if (shader) {
+        material->m_reserved = shader;
         return shader;
+    }
 
 #ifndef QSG_NO_RENDER_TIMING
     if (qsg_render_timing  || QQmlProfilerService::enabled)
@@ -489,6 +495,7 @@ QSGMaterialShader *QSGContext::prepareMaterial(QSGMaterial *material)
 #endif
 
     shader = material->createShader();
+    material->m_reserved = shader;
     shader->compile();
     shader->initialize();
     d->materials[type] = shader;