This helps select configs without alpha component.
Also fixed handling of EGL_DONT_CARE and EGL_BUFFER_SIZE.

Bug 21538709

Change-Id: I432a71e5df2a0da19a0c38195edf6c42c2d8b9aa
Reviewed-on: https://swiftshader-review.googlesource.com/3370Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>