Things like brush size and strength accessors now take a scene
parameter rather than guessing about which Scene's unified paint
settings to use.
Setting the size/strength through RNA can now be done separately for
the brush or the UnifiedPaintSettings.
The UI python code required updating to check whether the
size/strength controls should use brush or UnifiedPaintSettings RNA.
Radial control also required some updates to switch between the two
RNA sources.
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