- GLScatterPlotItem can work with arbitrarily-shaped vertex arrays
- added colorToAlpha function

Bugfixes:
- GraphicsScene emits sigMouseClicked for all clicks
- CanvasItem emits transformation change signal when mirrored
- GLViewWidget.pixelSize correctly handles position specified as array
- SRTTransform3D assumes Zscale=1 when converting from 2D transform