Add support for GL_EXT_shader_framebuffer_fetch

[s-inagaki923]

WIP. I'm reading the source code to implement this feature.

[s-inagaki923]

@kakashidinho
I don't understand most of it yet.

Can it be implemented by simply modifying the code in this repository?
Do you have any ideas on how to implement this? (For example, files that you think will need to be modified.)

I would appreciate it if you could tell me about it.

[s-inagaki923]

It seems that glslang does not support framebuffer fetch and I need to modify it.

[s-inagaki923]

I noticed that there is a way to convert gl_LastFragColor and gl_LastFragData to subpass input of GLES for Vulkan.

"--msl-framebuffer-fetch" argument of spirv-cross is:

   [--msl-framebuffer-fetch]:
           Implement subpass inputs with frame buffer fetch.
           Emits [[color(N)]] inputs in fragment stage.
           Requires an Apple GPU.

[kakashidinho]

Sorry, I was busy with other stuffs lately. I believe adding GL_EXT_shader_framebuffer_fetch support, you will need to modify the current metal back-end logic as well. Not just shader compiler.
Haven't had time to take a closer look yet, but I will try when I have time later.

[s-inagaki923]

Thank you!

[s-inagaki923]

Now, it should work on the actual iOS device with framebuffer fetching and the iOS simulator without framebuffer fetching.

[kakashidinho]

Thanks for this awesome PR.

I will put it on hold for now, I will need to test it more before merging to master.

[s-inagaki923]

I understand. I am looking forward to it.