Framebuffer completeness, usually verified through
glCheckFramebufferStatus, should return
GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE when the number of
samples is not the same for all attached renderbuffers.

Change-Id: I8d29895c6d4ded26847ce47fbd2581bbf06ed1bf
Reviewed-on: https://swiftshader-review.googlesource.com/4363Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>