mesa/st: short-circuit glFinish calls on WIN32 only

mesa/st: short-circuit glFinish calls on WIN32 only

Windows opengl32.dll calls glFinish prior to every swapbuffers, which
makes it pretty hard to get decent performance...

Work around by mapping finish to flush on PIPE_OS_WINDOWS.  This is
conformant, though it might confuse poorly-written benchmarks which
attempt to measure a single event rather than figuring out the rate of
continuous processing.