Add CAGradientLayer, Allow Layer Flattening for proper masking and opacity compositing

[ephemer]

Type of change: Feature

Motivation (current vs expected behavior)

• Add a simple CPU-only (for now) CAGradientLayer and the beginnings of a GPU implementation.
• Add layer flattening
  • With this, non-trivial layers with non-zero opacity and/or a mask are rendered as a separate render tree and then composited onto the window (or into each other)
  • This enables proper masking, e.g. via a gradient layer (via alpha) or using non-trivial shapes