failFlag=
AlphaAdditionTest([screenNumber])
Combine two alpha layers by addition and verify the result.
If no return argument is provided, then
TestAlphaAddition issues an error
when a test fails. If a return argument is supplied then it signals a
failed test only by returning true, without issuing an error.
See also:
AlphaBlendingTest,
PsychAlphaBlending
Psychtoolbox/PsychTests/AlphaAdditionTest.m