OpenGL ES的扩展: APPLE_texture_format_BGRA8888
http://www.khronos.org/registry/gles/extensions/APPLE/APPLE_texture_format_BGRA8888.txt
This extension introduces BGRA_EXT as an acceptable external format.
This avoids byte swizzling when loading RGBA internal format
textures, which may be stored in BGRA order internally.