Improve comments.
Clean up blend-mode code a bit.
Fix offset in direct blitting of nested Clip items.
Fix a bug with wrong offsets when rendering nested clips and blend-modes.

And streamline blend-mode and opacity rendering code.
Take strokeWidth into account in Item#hasStroke()
Add support for direct drawing of native blendModes for items that allow it.

Bypassing compositing into separate canvases completely.