[Qgis-developer] [WIP] Live layer "mask" effect

[Nyall Dawson]

Announcing a WIP "mask" effect for layers. This is a new layer effect
(alongside the existing drop shadows, blur, etc) which allows for
modifying the alpha channel of a rendered symbol/layer. Options
currently include gradient style masks (eg fade from top, fade from
left) and source in/out style composition masking.

I'm hoping to add a "mask from image" and "mask from symbol" option
before merging this.

Nyall