Add a layer and filter interface in the 2D canvas

This adds new beginLayer and endLayer functions to open and close layers
in the canvas. While layers are active, draw calls operate on a separate
texture that gets composited to the parent output bitmap when the layer
is closed. An optional filter can be specified in beginLayer, allowing
effects to be applied to the layer's texture when it's composited its
parent.

Tests:
 https://github.com/web-platform-tests/wpt/tree/master/html/canvas/element/layers
 https://github.com/web-platform-tests/wpt/tree/master/html/canvas/offscreen/layers

Fixes whatwg#8476