Refactor source interface to better accommodate non-tiled sources

We currently shoehorn all source types into the VectorTileSource interface by abusing Tile objects. For example, ImageSource populates a single Tile with a number of ad-hoc properties. This architecture breaks and causes bugs like https://github.com/mapbox/mapbox-gl-js/issues/3010.

There are several different types of sources in GL JS and each should have its own interface:

interface VectorTileSource {
    getVectorTileCoordURL(coord: TileCoord) => string;
    getVectorTile(url: string, callback: (error: Error, tile: VectorTile) => void) => void;
}

interface RasterTileSource {
    getRasterTileCoordURL(coord: TileCoord) => string;
    getRasterTile(url: string, callback: (error: Error, tile: ImageData) => void);
}

interface ImageSource {
    getImage(callback: (error: Error, image: ImageData) => void);
}

interface VideoSource {
    getVideo(callback: (error: Error, video: () => ImageData) => void);
}

Wiring up these interfaces to StyleLayers, SourceCache, etc will require some elbow grease and will be worth it. 💪 Note the elimination of Tile objects per #145.

Inspired by https://github.com/mapbox/mapbox-gl-js/issues/3010#issuecomment-244201644

Related to #2982 #281 #3010 #3123

Start with js/source/image_source.js at the linked lines, then trace how StyleLayers and SourceCache consume source and Tile objects. Review the related issues, especially #3010 and #145, before mapping the existing source types. Done means distinct source interfaces are wired through the affected components without using Tile objects for non-tiled sources.