Optimize texture loading and remapping

[SamVanheer]

Currently texture loading and remapping is handled inefficiently. Textures are first converted from 8 bit indexed to 32 bit RGBA, then resized if power of 2 textures are enabled, and then uploaded.

If a texture requires remapping this process is repeated after first updating the palette.

This should be optimized by incorporating remapping into the initial loading process, and performing the format conversion at the same time as the resizing.