Selection: transform selections via bitmaps, not polygons

[dsizzle]

Fixes #621

Not sure how to test this one other than just making sure the translate, rotate, and scale selections work at least as well as before.

I tested with rectangle, ellipse, freehand, and "select all non-transparent pixels" selections, both on new, blank canvases and existing pngs like Walter_the_OS, with undoing and redoing and such.

Filed #625 for the one issue I found that is also present in master before this change.

[humdingerb]

Seems to work fine.

[dsizzle]

merged! thanks!