Fix incorrect rendering when zoom-dependent text-size is used with icon-text-fit

Our current approach to icon-text-fit does not account for zoom-dependent text-size. We read the value of text-size during symbol layout: https://github.com/mapbox/mapbox-gl-js/blob/5bebe1cd725e9af0c6be25928bdbde468bebdf61/src/symbol/quads.js#L72-L92

But, at render time, we don't scale the icon to match the text size at the current zoom level. For icon-text-size: both, we could solve this by disallowing the icon-size property to be set and just using the text-size data in the icon rendering pass.

I'm not sure what we should do to handle cases where icon-text-size is used in only one dimension, and icon-size is used for the other. Some permutations of this could be handled with an extra uniform, but, for example, {icon-text-size: 'width', icon-size: [... composite expression ...], text-size: [... another composite expression ...]} would entail providing additional vertex attribute data.

Start in src/symbol/quads.js at the linked symbol-layout section, then trace the icon-text-fit rendering pass described in the issue. Determine how zoom-dependent text-size and icon-size combinations are represented, including one-dimensional and composite-expression cases. Done means icon rendering matches the text size at the current zoom without breaking supported icon-text-size permutations.