Native bindings to the APPLE_fixed_alpha_channel_orders extension.

These selectors may be passed to CreateImage2D in the cl_image_format.image_channel_order field. They are like BGRA and ARGB except that the alpha channel to be ignored. On calls to read_imagef, the alpha will be 0xff (1.0f) if the sample falls in the image and 0 if it does not fall in the image. On calls to write_imagef, the alpha value is ignored and 0xff (1.0f) is written. These formats are currently only available for the UNORM_INT8 cl_channel_type. They are intended to support legacy image formats.