drawing, switch from build-up to wash mode

change the pencil brush to work in wash mode instead of build-up mode

Build-up and wash mode names come from Krita:
-build-up mode means applying opacity to each dab of the stroke stack, so opacity result is always fuzzy.
-wash mode means applying opacity to the full stroke, and compositing the result after, to have perfect opacity control.

TODO: make similar change for other brushes, and maybe refactor a bit in the process.