[tile mode] Fix variable symbols placement

[pozdnyakov]

This commit allows the variable symbols to cross the tile border only if their anchor is the first anchor from the text-variable-anchor list.

[riastrad]

I've tested this locally and it seems to be correcting the behavior we observed. I'm going to QA a bit more, and if I don't catch anything then I think this should be good to go.

@pozdnyakov are we planning to add a regression test for the edge case that triggered this? (If so, do you mind linking to the PR that's tracking that?)

[pozdnyakov]

@pozdnyakov are we planning to add a regression test for the edge case that triggered this?

@riastrad the existing all-anchors-tile-map-mode test already does it.. I'm now adding a bunch of new render tests for the tile mode (the new tests will have anchor visible to make the result more clear).

[pozdnyakov]

mapbox/mapbox-gl-js#9235 <- another test that would catch the issue in a more clear way

[alexshalamov]

lgtm, new render test was tested locally and would be included in a next gl-js bump