Fix part of #37087, brush curve becomes inverted when using the reset
curve button.

From the bug report it also seems that positive slope curve presets are
not always reset correctly. However I haven't seen any place where
positive slope curve presets are used. This will fix the initial brush
problem, however it might be good to investigate further if curve
presets are to be used elsewhere. It looks like the issue is related to
bezier curve calculation.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60791 954f8c5b-7b00-dc11-b283-0030488c597c