Screen/Linux: Remove Intel-SNA workaround - It didn't work around :-(

Doesn't have expected effect, probably due to other bugs in SNA. Causes
a complete graphics system hang instead.

So situation now (further testing pending) seems to be that the latest
2.20.16+ drivers work well if Option "TripleBuffer" "Off" is specified
in xorg.conf, and don't work in any meaningful way for us at all if the
option is omitted.

Also add some more references to help SyncTrouble - Linux section.
Screen for 32-Bit Octave on Linux rebuilt for testing.