Proof of concept patch for Wash Mode artifacts

[Dmitry Kazakov]

Hi!

One of the bugs I'm working on currently is Wash Mode bug, described by
Paul in the first part of the bug [0].

Technical part:

Actually, this is not a bug, but a flaw of the composition formula we use
in ALPHA_DARKEN composite op. So to fix this bug, we need to change the
formula (with all the obvious consequences). The problem is this formula is
not commutative, that is the order of application of dabs changes the
result dramatically.

The patch:

In the attachment you can find a patch which (should) fix this bug. It
would be nice if someone tested it and looked for some non-obvious
regressions in painting it introduces.

This patch implements:
1) Fixes the artifacts on the tail of the Wash Mode stroke

This patch breaks (known issues):
1) Flow option doesn't work (it is simply disabled)
2) It fixes the bug for a Pixel Brush only
3) It disables vector optimizations


[0] - https://bugs.kde.org/show_bug.cgi?id=320651

-- 
Dmitry Kazakov
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