The display format does not have to match the framebuffer image format,
so we only care about the latter.

Bug 18510357

Change-Id: Ie3382b7b006a5007e56ff9e2ae572e8a60f500d8
Reviewed-on: https://swiftshader-review.googlesource.com/1572Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>