Use `enum EImageFormat` type for image format literals and variables.
Add `PixelSize` function to get the number of bytes/color channels per pixel for a specified image format.
Remove unused store format argument of texture loading functions. All textures are automatically being stored as RGBA, so the argument was unused. Also remove the therefore unused `FORMAT_AUTO`.
Rename variables consistently to `PixelSize` and use `size_t`, instead of mixing different names like `BPP` and `ColorChannelCount`.
Validate image format loaded from maps using `CImageInfo::ImageFormatFromInt`. Add `FORMAT_ERROR` to represent invalid formats.
Remove redundant `PixelSize` parameter from graphics backends and commands, which can be derived from the texture format.
Fix memory leak when RGB image data is being converted to RGBA format when saving map in editor.
