Locally generated glyphs shift on over-zoomed tiles

[asheemmamoowala]

When zooming from z17 to z18, I am seeing some locally generated glyphs being replaced by server retrieved glyphs. This is demonstrated in the gifs below. The second gif shows the box/size of the glyph made by adjusting the fragment shader.

On z18 tiles, the glyphs have variable size which is a property of the server generated glyphs. The z17 tiles have fixed size (24/32 px sq.) size that is characteristic of locally generated glyphs.

mapbox-gl-js version: v1.2.1

browser: Chrome

Steps to Trigger Behavior

1. With the local debug server open http://localhost:9966/debug/debug.html#18.04/35.681234/139.770108/-0.1
2. Zoom between z17 and z18

Link to Demonstration

Expected Behavior

Locally generated glyphs are not overridden by server generated ones.

Actual Behavior

At z18 server glyphs are taking over. Causing a change in the baseline and font of some characters.

cc @ansis @chloekraw @tristen