This is rather a workaround to avoid main thread freeing all glyph caches
at the same time as sequencer uses fonts to draw text sequences.
Ideally we need to either make cache more local, or user-counted or to make
somewhat more global locks. All this ends up in a bigger refactor which is
better for 2.8. For the meantime let's make Blender more stable with a tiny
workaround.
Downside is that keeping zooming things up and down in interface during render
will increase memory usage by unused glyph caches. It's not too bad though,
all unused caches will be freed first time at area zoom after render.
Thanks Bastien for review!
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