Screen: Fix multisample texture support for good.

Testing on Linux showed that the multisample textures can't get
used by the fixed function pipeline! The only way to access texture
data, apart from a glBlitFrameBuffer() call, is via binding of a
special texel fetch shader.

Implement support for such a fetch shader for "manual" multi-sample
resolve during texture blitting in PsychBlitterIdentity(). This is a
bit of a hack, like all things multisample texture related. However,
they need special treatment and they are only sampled from within the
panel fitter support code, either for rotated blits or as a fallback,
so i guess this hack is ok.

-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> Screen mex not included in this commit, to not interfere with
   release process.
-> Will not rebuild all Screen mex files before release. Too much
   work for the benefit of such a fringe feature. This improvement
   will be rolled out with the beta release after the upcoming release,
   ie., when the mex files get rebuild the next regular time.

-> Now that multisampled framebuffer rotation has been successfully
   tested, remove the test code in PanelFitterDemo.m before the release.