Prototype text run rendering optimization in WR

[ktaeleman]

We could render text runs as render task cache items, and draw into the main scene via the image shader (with dual source blending enabled as required).

This is likely to be slightly slower on GPU time, but have a number of positive flow on effects on other parts of WR (each one dependent on the last):

• Significantly reduced vertex count
• Less time / memory writing instances per-frame
• Feasible to make instances larger, embedding more information in them
• Reduce size / usage of vertex data and gpu cache textures

The first step is to prototype this in the easy (and common case), where the text runs have no weird transforms and/or raster scales. That will be enough to prove the concept, and do some initial measurements.

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🐞 Issue is synchronized with Bugzilla Bug 1639346