Added CCS filter, fixed unclosed layers and replied to review comments

This update addresses 3 main things:
- Support for CSS filter strings was added to the beginLayer API
- Unclosed layers are now never rasterized when the canvas is presented
  to the user. Instead, the content of the layer is preserved and will
  be rasterized in a later frame, if/when the layer is closed.
- Replied to the first round of review comments.