PsychGPUTestAndTweakGammaTables: Make more robust against buggy graphics drivers.

Move start y-pos of encoder test bar form 10 to 8, so 10-scanlines high
bar goes from -2 to 8. If a graphics driver (e.g., a buggy MS-Windows
driver) has a off-by-one or off-by-two y-pos bug, this should prevent
a black line on top of the screen and make this robust.

Also remove try-catch protection around Screen('LoadNormalizedGammaTable').
Was meant to protect against Windows gamma table upload failure but doesn't,
as Screen will close the window on such an error and code will continue to
run and spill out hundreds of misleading error messages.