Agata Cacko authored Feb 20, 2021 and Halla Rempt committed Mar 01, 2021

Before this commit, the oilpaint filter was broken: it would show tiling
sometimes and when painting on a layer with an oilpaint filter mask
attached, it would tile very badly around the brush (I was using
Wet Textured Soft for testing).
This commit fixes this behaviour by ensuring proper handling of opacity
and by changing usages of rawDataConst to oldRawData().

After changes, the results are roughly the same as before changes,
except that there is no tiling visible and painting is possible.
However the results are still quite much worse than in the
plugins/filters/tests/kis_all_filter_test expected results
would suggest. On the other hand, Krita's and Digikam's results are very
similar, too (and Digikam's results look wrong since they have random
black pixels in various places; Krita don't have them).

The problem of tiling was first noticed by SafariMonkey which also
contributed with first version of the patch (without handling of
transparency).

(cherry picked from commit 3891b207)