gui: Increase to 3 decimals on image upper/lower spin boxes.
doc: Add a couple of reference reading links.
glstate: Dump depth textures as floating point.
glstate: Dump all layers of framebuffers when GS is bound.

Fixes issue #297.
glretrace: Handle GL_ARB_sync properly.

When replaying, glretrace must wait for all fences that were recorded
as signalled in the trace, as subsequent calls might depend upon it.