Callback-based canvas source (or layer) API

Nobody has claimed this yet.

[CanvasSource](https://www.mapbox.com/mapbox-gl-js/api/#canvassource) allows users to create raster layers that use the contents of an arbitrary HTML Canvas element. However, like [ImageSource](https://www.mapbox.com/mapbox-gl-js/api/#imagesource), this API is currently oriented towards drawing into the fixed geographical area (determined by the `coordinates` option). This works well for drawing (possibly animated) graphics in a specific place, but it's not well suited to drawing graphics that "follow" the map, responding in realtime to camera movement.

To that end, I tentatively propose that we provide an API that allows users to provide a `render: (CameraState) => void` callback in addition to an HTML Canvas element. Our rendering pipeline would call the callback on every frame before reading data from the canvas. The canvas data would be assumed to be coincident with the map's viewport.

We could expose such an API by:

1. _Extending the `CanvasSource` API:_ Allow users to specify _either_ `coordinates:` _or_ `render: (CameraState) => void`.
2. _Adding a new source type:_ Like 1, but make it a separate source type.
3. _Adding a new layer type:_ Options 1 and 2 treat the canvas as a _source_ that can be targeted by `raster` layers. Instead, we could expose a `CanvasLayer` that provides the canvas element and `render` callback.

Start by reviewing the CanvasSource and ImageSource APIs, raster layers, and the rendering pipeline described in the issue. Compare the proposed CanvasSource, new source type, and CanvasLayer approaches, then define which API should support a CameraState render callback before canvas data is read on each frame.

Written by the indexing model from the issue text.