st/wgl: fix double-present on swapbuffers bug

st/wgl: fix double-present on swapbuffers bug

The stw_st_framebuffer_present_locked() function was getting called
twice per SwapBuffers.  First, when st_context_iface::flush() was
called from DrvSwapBuffers() because the ST_FLUSH_FRONT flag was
given.  Second, by stw_st_swap_framebuffer_locked() which does the
actual SwapBuffers.

Two code changes:
1. Pass ST_FLUSH_END_OF_FRAME, instead of ST_FLUSH_FRONT.
2. Move the implementation of stw_flush_current_locked() into
DrvSwapBuffers() since it's not called anywhere else.

Not much change in perf for benchmarks like Lightsmark, but some simple
Mesa demos are measurably faster.

Reviewed-by: José Fonseca <jfonseca@vmware.com>