Fix garbled text on zoom

[mourner]

Fixes #3669. Sorry for regressing this. The bug happened when different tiles of the same symbol layer had different fontstacks, which can sometimes happen during zoom when only a part of tiles is parsed before rendering.

I have no idea how to add a regression test for this though because it only happens intermittently and depends on a lot of factors like network speed, etc. So I suggest merging this without a test for now.

frame-duration benchmark shows no difference.

[lucaswoj]

I have no idea how to add a regression test for this though because it only happens intermittently and depends on a lot of factors like network speed, etc. So I suggest merging this without a test for now.

There have been so many untestable regressions lately. 😞 It might be time to invest in some test suite infrastructure for controlling network requests & the clock. I'll open some tickets.

[mourner]

@lucaswoj alternatively, we could add a semi-private API for explicitly loading a set of specified tiles and bypassing the source-cache pyramid logic; this way we would also be able to more easily test overscaled tiles, etc.