glamor: Improve error propagation and fallback paths when running out of GL memory

Some systems such as the Raspberry Pi have limited GL memory, which can easily be filled up and result in GL out-of-memory errors in glamor. In such cases, a SEGFAULT would often happen because of a FBO alloc return code not being propagated on error. This is fixed by the first patch of this series. With it, X keeps running and no longer crashes.

The following patches improve error handling and add support for a PBO fallback path.

With that, I can still see some glitches (e.g. missing text after resizing a window) when running out of GL memory, so there is still room for improvement.