use corner convention everywhere

all interpolators use corner convention
redo im_affine as a class
now runs!
now passes valgrind
im_affine*() compat functions work too
half-way through expanding transform

started putting in input displacement
can now displace affine input space

so hopefully that means you can change corner vs. centre convention
easily
deprecate vips7 resample dispatch
sync
Merge remote-tracking branch 'origin/master' into corner

Conflicts:
	ChangeLog
still segving with labq
Merge remote-tracking branch 'origin/master' into corner
fix up affine

now passes valgrind with both vips7 and vips8 interfaces