DrawFormattedText(): Allow centering of text inside user-provided rectangle.

So far, only centering in the target window/texture was possible, but
not wrt. arbitrary bounding boxes. Add an optional 11th parameter to
specify the rect into which text should be centered, clipped etc.
Merge pull request #114 from kleinerm/master

Pull new DrawFormattedText() improvement.
Merge pull request #115 from Psychtoolbox-3/master

PTB BETA RELEASE "Rainbows, Droids & Compute Hippies!" SP2
Add improved DrawFormattedText() with ability to center text in user-provided rects.