intel / DRI2: Additional flush of fake front-buffer to real front-buffer

To maintain correctness, the server will copy the real front-buffer to
a newly allocated fake front-buffer in DRI2GetBuffersWithFormat.
However, if the DRI2GetBuffersWithFormat is triggered by glViewport,
this will copy stale data into the new buffer.  Fix this by flushing
the current fake front-buffer to the real front-buffer in
intel_viewport.

Fixes bug #22288.
1 file changed