Implement tiling of canvas into smaller pieces

[yurydelendik]

The problem is that large canvases take much memory space. It's visible if a PDF page is large (e.g. map) or zoomed in (e.g. at 800%+ zoom). Currently we are limiting canvas size (#4834) for mobile device. However a proper solution will be to divide page into smaller canvases and render only visible parts.

It's mostly blocked by generating operator list based on crop area (useful for zooming heavy maps), but we can proceed without it, and try to render the same operator list on several canvases.

[yurydelendik]

(We might also consider to tile JPEG images when we read operator list)

[ManasJayanth]

@yurydelendik Can you help me get started on this?

[yurydelendik]

@prometheansacrifice first thing will be to modify API (and canvas.js) at https://github.com/mozilla/pdf.js/blob/master/src/display/api.js#L834 to have e.g. targets property which will be alternative to canvasContext. It will be an array of objects with canvas and location properties, which will define location of the canvas on the grid of canvases (Notice that now we will use CANVAS vs its 2d context to know its size; we might add canvas property along with canvasContext). canvas.js shall render now on multiple contexts at the same time.

[yurydelendik]

TODOs:

• modify API to render on multiple canvases and know their boundaries
• modify viewer to not create huge canvases (but multiple) and determine visible canvas tiles vs page
• provide boundary to getOperatorList and discard objects/text from the list that are not in the boundary