vrend: call glFlush after glFenceSync.

I hit app stall frequently when running virgl on Windows/macOS, it
turned out virgl kept calling glClientWaitSync and just got timeout.
According to https://www.khronos.org/opengl/wiki/Sync_Object, the sync
object may never be signaled without proper flushing. This glFlush fixes
the stall issue on Windows/macOS.

Tested-by: Jakob Bornecrantz <jakob@collabora.com>
Reviewed-by: Jakob Bornecrantz <jakob@collabora.com>
Signed-off-by: Lepton Wu <lepton@chromium.org>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/vrend_renderer.c

@@ -6062,6 +6062,7 @@ int vrend_renderer_create_fence(int client_fence_id, uint32_t ctx_id)
    fence->ctx_id = ctx_id;
    fence->fence_id = client_fence_id;
    fence->syncobj = glFenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 0);
+   glFlush();
 
    if (fence->syncobj == NULL)
       goto fail;