Use Workers to do tile decompression and unpremultiply

[Cwiiis]

• Resolves: #
• Target version: master

Summary

Two patches; the first fixes a bug where if we receive new keyframe tiledata and need to ensure the canvas, we end up decompressing/unpremultiplying/putting the old data unnecessarily (I think). The second uses a worker to offload the majority of tile image data decompression and unpremultiplying from the main thread and improves response during large update operations (such as scrolling through a document).

The code is not properly formatted yet, I'm opening this PR now and will update it with fixes once I figure out what those fixes are...

Checklist

• [x] Code is properly formatted
• [x] All commits have Change-Id
• [x] I have run tests with make check
• [x] I have issued make run and manually verified that everything looks okay
• [x] Documentation (manuals or wiki) has been updated or is not required

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[Cwiiis]

I'm going to close this as I have a newer, better version of the patch in a different branch. I'll likely need advice resolving some of these comments - I'll try to flag these issues explicitly.