Stretchable icons

[kkaefer]

Fixes #16017

Implements stretchable icons, porting from JS.

[kkaefer]

Remaining questions:

• We're using std::vector for storing stretches, which might be using quite a bit of memory. Ideally, we'd use a "small vector" that stores 0-1 elements inline to avoid heap allocations.
• We're storing texture coordinates as integers in Native. I believe we do that as well in JS, which means that we'll have to remove the option to pass floating point numbers in stretches.

[ansis]

We're storing texture coordinates as integers in Native. I believe we do that as well in JS, which means that we'll have to remove the option to pass floating point numbers in stretches.

Yep, should we do something to enable some fractional values? I could see half pixel values being useful but not sure about anything beyond that.

[alexshalamov]

looks great! few nits

[ansis]

I took a look a look at the core logic part and I checked that all the followup fixes I made in -js are also included here. Those look good (assuming the TODOs are done). I'll leave the rest of the review to Alex

[kkaefer]

@alexshalamov thanks for rebaselining!

We're using a lot of std::vectors that have no or only one element. my fear is that std::vector is using too much memory + heap allocations. Is there a tutorial for how I could benchmark this in GL native? I'm looking at https://github.com/abseil/abseil-cpp/blob/master/absl/container/inlined_vector.h; have we considered including abseil (or some other library) that would give us these types?

[alexshalamov]

@kkaefer I created an issue #16118 to follow-up idea of using inlined vectors.