The EXT_read_format_bgra extension specifies that GL_BGRA_EXT can only be
returned for GL_UNSIGNED_BYTE, UNSIGNED_SHORT_4_4_4_4_REV_EXT, and
UNSIGNED_SHORT_1_5_5_5_REV_EXT types.

Bug swiftshader:38

Change-Id: I54d7b6bbb0954fce87ee7622207fe32a308a43d9
Reviewed-on: https://swiftshader-review.googlesource.com/9030Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>