Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
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.
Sponsored by hundreds of generous kickstarter contributors!
