various: pixel optimizations

Several things are accomplished in this commit, including:

- Use OIIO instead of our own functions for pixel format conversions
  (cleaner code/less for us to maintain)

- Fold all PixelService functions into the PixelFormat class
  (cleaner code)

- Moved OpenGL pixel definitions to OpenGL classes and out of the
  global classes.

- Add support for RGB buffers as well as RGBA (optimization)

app/render/backend/opengl/openglworker.cpp

@@ -7,7 +7,7 @@
 #include "openglcolorprocessor.h"
 #include "openglrenderfunctions.h"
 #include "render/colormanager.h"
-#include "render/pixelservice.h"
+#include "render/pixelformat.h"
 
 OpenGLWorker::OpenGLWorker(VideoRenderFrameCache *frame_cache, DecoderCache* decoder_cache, QObject *parent) :
   VideoRenderWorker(frame_cache, decoder_cache, parent)