Fix flaky text-variable-anchor render tests

[mourner]

text-variable-anchor/pitched-with-map render test failed in a commit that just changed the changelog, so it's apparently flaky (it passed after rerunning the tests) — let's investigate. cc @alexshalamov

[alexshalamov]

@mourner Just tried running gl-native tests

for i in {1..20}; do build/linux-x86_64/Release/mbgl-render-test text-variable-anchor -r --shuffle --seed=$RANDOM; done

Really strange that it is flaky on gl-gs, we had few flaky tests in native, as the rendering results were created in gl-js and native uses floats in placement code (double in gl-js).

[mourner]

Another example of a flaky test, this time it's text-variable-anchor/pitched — the pattern of failure is very similar:

[kkaefer]

Got another instance of this. @arindam1993, @alexshalamov let's fix this.