Screen/Linux: Fix X11 display locking more.

There was a hidden locking error when running on the FOSS
(Mesa/Gallium) graphics drivers under X11: Each scheduled
doublebuffer-swap will invalidate the drawables system framebuffer.
First access to the fb by OpenGL will therefore trigger a roundtrip to
the X-Server for revalidation. This uses our shared x-display
connection handle for transport and therefore needs lock protection in
multi-threaded ops like frame-sequential stereo of async flips. The
NVidia binary blob uses a different mechanism for revalidation which
bypasses the X-Server, so this problem didn't show up during
pre-release testing.

We can't lock protect any glXXX call just in case, that would be a
performance disaster. Therefore we perform a dummy pixel write +
glFlush() immediately after scheduling a bufferswap and we do this
under display lock protection. This write after bufferswap will trigger
the X11 roundtrip for fb revalidation under the lock. After that
validation we should be safe from such surprises until the next
bufferswap gets scheduled.

Hopefully the last locking bug from our locking rewrite, this is
painful.

-> Screen mex files for 32/64-Bit Matlab/Octave on Linux rebuilt.
-> Tested in VM, but this is mostly pointless.
-> Tests on both NVidia proprietary and nouveau FOSS pending.