The "spacing" option of the warp tool used to be handled by the
gegl:warp op, and have little effect.  Instead, implement it in the
warp tool directly, having the same effect as the other paint
tools.

Having a properly-working "spacing" option allows us to use EXACT
motion mode without cirppling down performance, which means that
the stroke now follows the pointer exactly, even when processing
takes a while.

Decrease the default "spacing" value to 10.