Looked a bit deeper into heal: while I didn't try to understand what
it's actually doing, this is strange: there is a comment that says
that healing should done in perceptual space, and the code uses
R'G'B'A float (at least it completely did before the last commit).

On the other hand, the code adds and subtracts temporary buffers,
which screams "gamma artifacts" unless done in linear space.

This commit changes everything to use linear float buffers,
and removes the comment. It "looks" right to me now, please test.