Run clang-tidy modernize-use-override to remove all the redundant
virtual keywords from overridden methods, and add some missing
overrides.
Another benefit is that this has also added the overrides
on destructors, which will cause a build failure if a base
class is missing a virtual destructor.
This commit adds a new framework for implementing paint effects, which
modify the results of QPainter operations to apply visual effects
such as drop shadows and blurs.
The initial implementation allows for effects to be applied to entire
layers and individual symbol layers.
Included are a drop shadow, inner shadow, blur, inner glow, outer glow,
colorise and transform effect. A "stack" effect is also implemented
which allows other paint effects to be combined in various ways.
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