subpixels: clientrect, getBoxQuads, getBBox etc allow high precision measurements which can expose subpixels

links

old issue: #29564 (closed) (nothing to see, it's all in this description)
upstream: 1507879
example of methods: https://canvasblocker.kkapsner.de/test/domRectTest.php

We can control a lot of measurement entropy via other means per platform: such as unified look and feel, default fonts + sizes per language, controlling available fonts, etc. This ticket is not about those. This is about possible subpixels

All these methods, and we could add textMetrics to the list, can be subtlely randomized now Firefox has a seed per eTLD+1 per session .. THIS. IS. THE. WAY. (after standardizing everything else).

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