Screen: Make more robust against bogus multisample reports from drivers.

The Tegra-3 driver claims multisample is 1 when multisampling is
disabled. This throws off code that later on check for zero == off.

Force value to zero if it isn't greater than 1 to make this more
robust and fix problems with panelfitter on Nexus.

Also enable use of imaging hook for 'UserDrawingFinished', which was
defined since years, but not hooked up to anything. This allows to
execute commands at Screen('Flip') or Screen('Drawingfinished') time,
immediately after usercode has finished its drawing and before any
other preflip operations. This works even with imaging pipeline disabled.

Use case: Drawing of some ptb generated overlay graphics on top of user
created stimulus image, shortly before any post-processing happens. E.g.,
this could do callback that draws a virtual onscreen keyboard on top of
the stimulus image. Caution: No calls into Screen() allowed!