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/codec/oiio/oiiodecoder.h

@@ -24,7 +24,7 @@
 #include <OpenImageIO/imageio.h>
 
 #include "codec/decoder.h"
-#include "render/pixelservice.h"
+#include "render/pixelformat.h"
 
 class OIIODecoder : public Decoder
 {
@@ -57,7 +57,7 @@ private:
 
   PixelFormat::Format pix_fmt_;
 
-  PixelFormat::Info pix_fmt_info_;
+  OIIO::TypeDesc type_;
 
   bool is_rgba_;