Fix validation typo
New filter support function for forcibly erasing pixels

I needed this for the new "spherize" distort
Infrastructure for new distort tools (new icons, menu layouts, etc)
New "Poke" distort

Different from whirl, so worthy of inclusion, I think.  GIMP relies on
an algorithm like this for its own lens distort correction - basically,
corners are bent backward and the center is pushed outward, per a
variable strength setting.  Can also be used stylistically.
New "Spherize" distort

Similar to the matching function in PhotoShop.  Rotation and
horizontal/vertical offsets are supported, as well as a fun "light ray"
technique I couldn't resist.  :)