rusticl/mesa/context: flush context before destruction

Drivers might still be busy doing things and not properly clean things up.

Fixes a rare crash on applicatione exits with some drivers.

Fixes: 50e981a050 ("rusticl/mesa: add fencing support")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29223>

src/gallium/frontends/rusticl/mesa/pipe/context.rs

@@ -659,6 +659,7 @@ impl PipeContext {
 
 impl Drop for PipeContext {
     fn drop(&mut self) {
+        self.flush().wait();
         unsafe {
             self.pipe.as_ref().destroy.unwrap()(self.pipe.as_ptr());
         }