Improved support

• Refactored mipmap loading to read in reverse order (largest->smallest). This resolves a rare OOB error caused by an off-by-8bytes offset into mipmap data. This could be reproduced most easily with all 6 faces of the map de_dust2's skybox textures.
• Added quality of life functions to more easily fetch the 'first' texture (highest-resolution, first frame mipmap).
• Made return types for image data uniform (replaces instances of []byte with []uint8). These type changes are won't affect compatability.