Native bindings to the IMG_read_format extension.

This extension is intended to supplement the GL_OES_read_format extension by adding support for more format/type combinations to be used when calling ReadPixels. ReadPixels currently accepts one fixed format/type combination (format RGBA and type UNSIGNED_BYTE) for portability, and an implementation specific format/type combination queried using the tokens IMPLEMENTATION_COLOR_READ_FORMAT_OES and IMPLEMENTATION_COLOR_READ_TYPE_OES (GL_OES_read_format extension). This extension adds the following format/type combinations to those currently allowed to be returned by GetIntegerV:

    format                      type
    ------                      ----
    BGRA_IMG                    UNSIGNED_BYTE
    BGRA_IMG                    UNSIGNED_SHORT_4_4_4_4_REV_IMG

E.g. Calling GetIntegerv with a pname parameter of IMPLEMENTATION_COLOR_READ_FORMAT_OES can now return BGRA, with the corresponding call to GetIntegerv using a pname parameter of IMPLEMENTATION_COLOR_READ_TYPE_OES returning UNSIGNED_BYTE;