Selection->Rotate is incorrect

[dsizzle]

Selections in ArtPaint are stored as bitmaps, and then a polygonal representation of the bitmap is displayed to the user as the selection boundary.

When selections are rotated, the polygonal representation is rotated:

This is not correct because the "pixels" on the rotated selection no longer match the pixels of the image, and this can lead to ugly artifacts.

Instead, the underlying pixels of the selection should be rotated, and then the polygonal representation should be updated to match.

[dsizzle]

Closed via #621