separation of previous renderer into base and derivation complete

The previous iteration was fairly OpenGL-heavy. It's now been separated into
a base class that is OpenGL independent and a derived class that is
OpenGL-based. Over time this should allow for portability away from OpenGL
if necessary.

app/core.cpp

@@ -317,7 +317,7 @@ void Core::DeclareTypesForQt()
   qRegisterMetaType<Task::Status>("Task::Status");
   qRegisterMetaType<NodeDependency>();
   qRegisterMetaType<rational>();
-  qRegisterMetaType<RenderTexturePtr>();
+  qRegisterMetaType<OpenGLTexturePtr>();
 }
 
 void Core::StartGUI(bool full_screen)