qapitrace: Support executing glretrace on a remote target host.

This is supported with a new command-line argument:

	qapitrace --remote-target <HOST> <trace-file>

See README.markdown for documentation on usage and current limitations
of this feature.
gui: Fix includes.

stdio.h doesn't seem necessary. stdlib.h might be though, for exit().
Cleanup README formatting.
glretrace: Flush outstanding requests when waiting for user input.

This helps make "apitrace replay -sb" work in cases where the driver
really only exposes double-buffered rendering, (so a flush is required
to make anything appear if the trace doesn't include a final
SwapBuffers).
replay: Add a --loop option to repeatedly loop over the final frame

This can be useful for repeatedly exercising the driver with the
operations of a single frame.