Bug 827170 - Clamp intermediate surface's framebuffer dimensions to what is supported by the GL - r=BenWa, a=bajaj

Without this, we have assertion failures as we fail to create our textures and subsequently
we have incomplete framebuffers. The present patch is a short-term compromise: to avoid
asserting, we just clamp texture sizes. That can result in fuzzy rendering. Ideally
(with some suitable tiling) we wouldn't have to do that.