Can we perhaps rewrite this during SKSL generation automatically?

Can we perhaps rewrite this during SKSL generation automatically?

Do you mean rewrite FlutterFragCoord() in the SkSL backend, but leave it alone and use the definition in this header otherwise? That would avoid the ifdefs for sure, or were you thinking of a change that would avoid the header entirely?

A couple of constraints to consider here:

• The sksl generator is actually hijacking gl_FragCoord and supplying a varying input to it. Not doing this will break all existing shaders. I think the consensus is that backwards compatibility isn't a huge deal right now, but is a nice-to-have.
• The source input shader must be valid GLSL for the compiler to accept it and translate to spirv. I don't think there's a way for us to define our own custom builtins and then rewrite them in spirvcross.

One thing I considered was just removing this gl_FragCoord hijacking in the sksl generator and just having users always specify the varying input -- this would break all existing shaders, of course. One of the reasons I ended up going for this opt-in backwards compatible header shim was that the runtime effect use cases (color source and filters) will only ever support a strictly managed attribute layout anyways, and so it feels like this solution ends up being simpler for users. They just get a function to call and don't have to worry about keeping the varying layout correct. A slight side benefit is that this also allows us to change the vertex layout without breaking users (although I can't imagine we'll need to).

This solution seemed to hit the sweet spot for me, but I'm totally open to other ideas even if it means introducing breakages, etc.