wintab32: Handle negative output context range for y-axe

[John Klehm]

Hello,

This fixes bug 11838 http://bugs.winehq.org/show_bug.cgi?id=11838
(probably some of the other inverted y-axe bugs too).  I've tested
against openCanvas 4.5 and Photoshop CS2 and they seem to have no
regressions.

Cheers,
--John Klehm
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