Draft: Issue #3247: Fix generated brush preview angle

This patch attempts to resolve issue #3247.

The Generated Brush angle range of 0 to 180 is half the Paint Tools angle range of -180 to 180. This causes the preview to be mirrored because the angle is off by -180.

This patch multiplies the preview angle by -1 to make the preview match the tool. It can also be easily backported for the upcoming 2.10.36.