[Impeller] Add testonly STB config

[bdero]

For flutter/engine#44887.

[zanderso]

I wonder if we could find a place for this to live in flutter/engine instead of in buildroot?

[chinmaygarde]

We could put a tertiary build dir tree that in the engine itself.

[bdero]

This would be great. I added an issue about it here: flutter/flutter#132981.

If we move this build config into the engine tree, we should also move the other testonly deps I've added in the past like imgui and tinygltf, as they're in more or less the same situation.

[chinmaygarde]

We could put a tertiary build dir tree that in the engine itself.

[chinmaygarde]

We discussed this in triage. But build/secondary/third_party/stb/stb_truetype_stub.cc isn't vendored and can live anywhere and doesn't need to live in third_party. I think we can get rid of this and do it entirely in the the engine repo.

[bdero]

But that's what I was trying to address with the bug. Where should we start putting stuff like this in the engine repo? Maybe we could just throw it in as //flutter/third_party/stb:stb_truetype since we need to make the stub TUs anyway?

[chinmaygarde]

Yeah, this is sort of problematic with libraries that are header only but ask you to stamp out an implementation in exactly on TU by defining a FOO_IMPLEMENATATION macro. We have done this for VMA in flutter_vma.cc. VMA itself is vendored. But the one implmenetation of VMA is in a TU that resides in the engine. And hence its BUILD.gn file can live there too. Perhaps do something similar for STB? Perhaps in the Impeller typographer itself?