st/wgl: check if WGL_SWAP_INTERVAL is defined in wglSwapIntervalEXT()

st/wgl: check if WGL_SWAP_INTERVAL is defined in wglSwapIntervalEXT()

This allows the WGL_SWAP_INTERVAL env var to override any application
calls to wglSwapIntervalEXT().  Useful for debugging, or to set the
interval to zero to effectively disable the swap interval.

Note: we also rename the previous instance of SVGA_SWAP_INTERVAL to
WGL_SWAP_INTERVAL since this is a WGL feature and not related to the
svga driver.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>