st/wgl: pseudo-implementation of WGL_EXT_swap_control

st/wgl: pseudo-implementation of WGL_EXT_swap_control

This implementation is based on querying the time just before swap/present
and doing a Sleep() if needed.  There is no sync to vblank or actual
coordination with the GPU.  This isn't perfect, but basically works.

We've had some request for this functionality, and it sounds like there
are some Windows GL apps that refuse to start if the driver doesn't
advertise this extension.

Note: NVIDIA's Windows OpenGL driver advertises the WGL_EXT_swap_control
string both with wglGetExtensionsStringEXT() and with
glGetString(GL_EXTENSIONS).  We're only advertising it with the former at
this time.

Tested with asst. Mesa demos, Google Earth, Lightsmark, etc.

VMware bug 1591534.

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