Specifically, in case the source surface was offscreen, we would sometimes to
GL calls without active context.

The register number is already in the name we got from get_register_name,
don't add it twice.

Backbuffer appears to be a better choice than PBuffer at the moment, since
PBuffer appears to be somewhat broken.

Nvidia allows it to operate on a standard Window too but Mesa and AMD
don't and this isn't needed according to the GLX specs. This prevents
crashes when creating +wgl logs.