PTB BETA RELEASE "Random acts of kindness" Psychtoolbox-3.0.12 - SP4

* New Gaussblob procedural shader.
* New mini 4-channel mix shader.
* Demo cleanups and refinements.
* KbQueueReserve fix.
* Some DRI3/Present prep work for Linux - not relevant yet, only included in Screen for 64-Bit Octave on Linux so far.