Screen: Really don't use AUX buffers if disable requested.

The kPsychDisableAUXBuffers setting is honored in the sense that no AUX
buffers are requested. However, this equals requesting a visual with >=
0 AUX buffers, as zero is the default. On GPU's where all visuals have
> 0 AUX buffers this means we get an AUX buffer we didn't ask for. If
code later uses aux buffers if any available, it will use them despite
our disable. -> Check for no-aux-buffer request before use of aux
buffers, not only at context creation time.

This is a minor, practically irrelevant change, just needed for
debugging something. Therefore we won't rebuild mex files for it.
Screen/Linux: Improve buffer_age filtering.

EXT_buffer age queries work mostly as expected :-)

Filter out zero case --> Can happen due to driver optimizations, not an
indication of failure.
Value 1 --> CopySwap due to redirection due to compositor activity -->
Finally something that really tells us if a compositor is interfering
:-).
Value 2 -> Double-buffering -> everything good.
Value > 2 -> Triple/n-buffering: Fatal for timestamping.

Now we filter out non-fatal values and give some useful help for known
values 1,2,3,...,n.

Screen for 64-Bit Octave on Linux rebuilt and tested with latest NVidia
binary driver beta release. Works!