[rbp] Remove 720p GUI limit by default

The initial Pi port of xbmc limited the GUI to 720p.
This was mainly to save memory, although it has a small effect on performance.

Since the recent "speed" commits the performance is much better, and also the memory usage is much lower.
This was due to:
  ARM never owns pixel buffers of textures
  GPU doesn't require whole frames of decoded jpegs as it decodes/converts in stripes
  GPU artwork textures are stored as 16-bit

So, this commit allows the GUI resolution to specified in settings, and it defaults to 1080p.
I've also added a switch to allow the artwork textures be 32bit. This improves artwork quality, with some memory cost, but is viable on 512M devices.