Color3DLUTTest: Fix potential error on wrong window size.

‘GetImage’ for comparison read back assumed the window size would not
change. As we use a GUI window, user could change size and cause error
abort. Originally this was triggered by a off-by-one size bug on Linux
with some WM.

Make Screen(‘GetImage’) read back size explicit, so we don’t error out,
but may report wrong precision. Not using onscreen windows for this
test would be even better, but meh…
Screen/Linux: Fix off-by-one error in window size query for kPsychGUIWindow's

PsychOSProcessEvents() recalculates window size from queried results on
kPsychGUIWindows to account for live resizing of the GUI window by the
user. On Linux it erroneously subtracted 1 from width and height, when
width and height was already correct, leading to some failure in some
tests, because all code suddenly assumed a 1 pixel smaller window.
Fixed in this commit.