Eliminate `text-size`/`icon-size` evaluation special cases

Nobody has claimed this yet.

As noted in https://github.com/mapbox/mapbox-gl-native/issues/10462#issuecomment-344449031, the symbol layout process currently evaluates `text-size` at up to five zoom levels and `icon-size` at up to three:

1. `text-size` at the zoom level of the bucket. Used to calculate a per-feature size for source `text-size` expressions (1a), and to calculate the box dimensions for icon-text-fit (1b).
2. `icon-size` at the zoom level of the bucket. Used to calculate a per-feature size for source `icon-size` expressions.
3. `text-size` and `icon-size` at the zoom level of the bucket, plus one. Used to calculate collision boxes.
4. `text-size` at zoom level 18. Used for something line-symbol-placement-related, possibly to reduce label movement by placing line labels in the same spot at different zoom levels. (Institutional knowledge is hazy.)
5. For composite `*-size` expressions: two zoom levels of curve stops that "cover" the zoom level of the bucket. These go into a vertex buffer and are used by the shader to interpolate the size at render time.

(1a), (2), and (5) are the non-special cases -- all data-driven properties require them. https://github.com/mapbox/mapbox-gl-js/issues/5656 tracks fixing or eliminating (1b).

This issue focuses on (3) and (4). From conversations with @ChrisLoer and @ansis, the introduction of viewport placement likely allows us to eliminate them. We should do that.

Start by reading the symbol layout process and the viewport placement work referenced in the issue, then review mapbox-gl-js issue 5656 for related scope. Trace the zoom-level evaluations for collision boxes and line-symbol placement; done means those special cases are eliminated while the remaining data-driven and shader interpolation evaluations continue to work.

Written by the indexing model from the issue text.