[core] Evict unused font stacks from GlyphManager

[jfirebaugh]

Fixes #12388.

[ChrisLoer]

This looks like a good approach to me, but did you also consider something like an LRU eviction strategy? I haven't thought it through too much, but I guess pros would be:

• GlyphManager wouldn't be tied to being used with a particular style (or if you wanted to use it with multiple styles, you wouldn't have to keep track of the union of their fontstacks). Although right now the GlyphManager is tied to the style by the renderer, I can imagine us wanting to share a single GlyphManager w/ separate renderers (e.g. for MapSnapshotter)
• The Layer '%s' has an invalid value for text-font and will not work offline. error wouldn't have to turn into a hard error
• You might potentially use it to constrain memory use even in the single style case (imagine a multi-font map with lots of Chinese characters -- you could end up with a lot of data in memory, although without a disk cache, it would suck to discard the glyphs only to fetch them back from the network...)

Cons:

• Maybe more complicated book-keeping?
• Maybe doesn't match as well what you actually want to hold onto. e.g. if you're under the LRU cap but you've stopped using a fontstack, holding onto it is still a waste, and if you're over the LRU cap even with just one style, then maybe you'd rather use memory than discard glyphs.

[jfirebaugh]

LRU doesn't really feel like the right fit to me. We don't want glyphs to get evicted when they may still be required, and we don't want them to stick around when they're no longer needed.

Plus it would lead to a more complicated implementation, and we'd have to spend some time figuring out what a reasonable cache limit is.

[jfirebaugh]

This has been incorporated in the node v4 beta with success; let's merge it to master.

[ChrisLoer]

👍