Render a separate alpha mask:
This allows a generic optimization to skip all invisible pixels when
updating background. For triangle this alpha mask allows to skip ~50%
pixels that were never displayed.
Performance for ring shape stayed identical, was hoping for a small
boost at least...

Rework triangle code:
Removed some unused variables and use widget space a little better.
While at it, correct math that assumes certain widget geometry constraints.

TODO: code to fit triangle inside ring needs a tweak.
TODO: implement alpha mask for rectangle shape.