app, pdb, plug-ins: lower Spread limit for Distort

Select->Distort causes an error when Spread > 200, and it can be set up to 1000 in the plug-in dialog:

Plug-in 'Distort' left image undo in inconsistent state, closing open undo groups.

This limits Spread to 512 which is the highest allowed by gegl:noise-spread and raises the limit of corresponding x and y variables in the gegl call to 512.

Tested to work, a high portion of selection pixels get dropped for being outside the canvas for high Spread values, but that seems to be the expected behavior.