implemented texture cache for performance

Creating and destroying textures is a slow process, particularly when we can
re-use them throughout most of the render chain. We now keep them stored so
they can be re-used which improves performance substantially.

app/core.cpp

@@ -319,6 +319,7 @@ void Core::DeclareTypesForQt()
   qRegisterMetaType<NodeDependency>();
   qRegisterMetaType<rational>();
   qRegisterMetaType<OpenGLTexturePtr>();
+  qRegisterMetaType<OpenGLTextureCache::ReferencePtr>();
   qRegisterMetaType<NodeValueTable>();
 }