Font subset

[dnfield]

Adds a font subsetting utillity to the engine build.

This is meant to be consumed by the flutter tool to help with glyph shaking. Once it lands in tree we can update the recipes to upload it to storage along with the other host artifacts.

Tagged a bunch of people for review:

@zanderso and @jonahwilliams for evaluation of how this will be used by the tool

@jason-simmons @GaryQian @chinmaygarde because they probably know more about harfbuzz than I do, and general engine review.

@willlarche FYI (and feel free to give feedback as well).

[GaryQian]

Is there an issue this fixes for more context?

[dnfield]

Fixes flutter/flutter#43642

[dnfield]

I was expecting this to run the tests I wrote on all platforms, but it looks like not unfortuantely. That will require a recipe change, or that I make these tests run under the "engine" category for now. I'm going to push something up that does that at least temporarily to try to get them to run on Windows, since I haven't tested it there yet.

[jonahwilliams]

Seems like this would be fairly straightforward to incorporate into the asset build, provided that we already knew the subset of fonts needed? I'm assuming that later part is where the kernel parsing comes in

[dnfield]

Seems like this would be fairly straightforward to incorporate into the asset build, provided that we already knew the subset of fonts needed? I'm assuming that later part is where the kernel parsing comes in

Yes

[dnfield]

Having issues with the test. I'll look at it some more later. The script seems to try to compare before the process has finished writing files