Reduce slowness when loading many raster tiles at once

Nobody has claimed this yet.

We're bottlenecked on too many texture uploads occurring at the same time, and Webkit's timeline recorder confirms this:

It seems that uploading a single texture takes ~7.5ms per raster tile. On smaller screens, this is typically split up across multiple frames, but if you have a fast network and a big screen, many of the load events can happen within the same frame.

We should:
* [ ] Investigate why the calls to `texImage2D` and `generateMipmap` raster tile take a combined ~7.5ms. The tiles in the screenshot above `@2x` tiles (512×512), but even then ~7.5ms seems excessive.
* [ ] Distribute raster tile loads across multiple frames and upload at most 1-2 tiles per frame.
* [ ] Don't generate the mipmaps immediately after loading, but wait a frame to spread the load.

Further avenues for investigation:
* [ ] Check whether using `texSubImage2D` to upload the bitmap in batches, and spread the upload of large bitmaps across multiple frames improves the performance.
* [x] A [comment on the chromium mailing list](https://groups.google.com/a/chromium.org/d/msg/chromium-dev/0j74Nd7i7IA/NmYiPkClKAAJ) claims:
> Image decompression happens after the onload handler fires. [...] The ImageBitmap API will fix this problem.

The [ImageBitmap](https://developer.mozilla.org/en-US/docs/Web/API/ImageBitmap) API is supported in Chrome and Firefox at the time of this writing, but not Safari/Webkit, IE and Edge. It looks like we could use this to make image decoding asynchrous: The `createImageBitmap` constructor method returns a promise that will be resolve to a `ImageBitmap` object, so the call to `texImage2D` doesn't need to decode the image first. (See example in https://gist.github.com/ahem/d19ee198565e20c6f5e1bcd8f87b3408). Note that [at least Chrome](https://bugs.chromium.org/p/chromium/issues/detail?id=580202) still decodes on the current, thread, so we need to do this in a webworker.

Start with the Webkit timeline recorder and investigate the raster-tile calls to texImage2D and generateMipmap. Compare the current upload timing with spreading uploads across frames, deferring mipmap generation, and the possible texSubImage2D or ImageBitmap approaches. Done means raster tiles no longer upload more than 1–2 at once and mipmap work is spread across frames.

Written by the indexing model from the issue text.