wined3d: Flush the GL context before switching away from it.

We might end up not using the old context anymore after that and the GL
implementation is allowed to delay executing commands indefinitely if we
don't flush.

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

dlls/wined3d/context.c

@@ -1129,6 +1129,11 @@ BOOL context_set_current(struct wined3d_context *ctx)
         }
         else
         {
+            if (wglGetCurrentContext())
+            {
+                TRACE("Flushing context %p before switching to %p.\n", old, ctx);
+                glFlush();
+            }
             old->current = 0;
         }
     }