[rust png] Support Rust encoder + libpng decoder to build gm/image.cpp
This CL adds support for the following `args.gn`:
```
skia_use_rust_png_decode = true
skia_use_rust_png_encode = false
skia_use_libpng_decode = false
skia_use_libpng_encode = true
```
Using Rust encoder and libpng decoder is a bit hypothetical, but it is
sometimes useful to exercise this scenario when testing build changes -
this came up during Rust BMP work.
Bug: chromium:381900683
Change-Id: I39e46d76a3df50cc188ee53a8a5f808637c2abe2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1109497
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Łukasz Anforowicz <lukasza@google.com>
1 file changed
