Fix using an alpha-less GL format for Android's RGBX.

Bug 19979126

Change-Id: Ic074079cbe3630751a95ffcc2b2b33700063abcf
Reviewed-on: https://swiftshader-review.googlesource.com/2782
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Ping-Hao Wu <pinghao@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>