winex11: Only use glXSwapBuffersMscOML() for child windows.

The notion that glXSwapBuffersMscOML() with 0/0/0 target_msc/divisor/remainder
is supposed to behave like glXSwapBuffers() with respect to the swap interval
turns out to have been a misconception on my behalf, the extension spec
doesn't actually say that. Adding to the confusion is the fact that some
drivers do in fact behave like that in practice. It's possible to make
glXSwapBuffersMscOML() use the proper swap interval, but it's much easier to
just use glXSwapBuffers() instead. One consequence of the approach taken in
this patch is that child window buffer swaps aren't necessarily synchronised
to vblank, but ultimately that's up to X11DRV_FLUSH_GL_DRAWABLE, not the
buffer swap. This fixes a regression introduced by commit
e75bc6228e.

Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/winex11.drv/opengl.c

@@ -3356,7 +3356,7 @@ static BOOL glxdrv_wglSwapBuffers( HDC hdc )
         escape.gl_drawable = gl->window;
         /* fall through */
     default:
-        if (pglXSwapBuffersMscOML)
+        if (escape.gl_drawable && pglXSwapBuffersMscOML)
         {
             pglFlush();
             target_sbc = pglXSwapBuffersMscOML( gdi_display, gl->drawable, 0, 0, 0 );