[3.x] Fix fragcolor write locations in scene shaders

[lawnjelly]

This will need careful checking by @clayjohn as I'm not super familiar with all the parts of the scene shaders.

Fixes #91967

Discussion

In retrospect, I'm not totally surprised I got these in slightly the wrong place, the #ifdef arrangements especially in the scene shaders are very difficult to follow (manually). I would welcome suggestions for glsl editor that will follow them sensibly, geany can identify from opening, but not from closing. Other than that I'm relying on find_in_files.

What makes things particularly difficult is that the comments in the scene shaders appear to be out of sync:

// scene.glsl line 2464
#endif //RENDER_DEPTH //ubershader-runtime

presumably should be not RENDER_DEPTH.

This appears to have happened several times in the shaders because files originally contained e.g.

#ifdef RENDER_DEPTH
#endif // RENDER_DEPTH

then an #else was inserted, and the ending comment became incorrect.

I suspect these shaders could do with a second pass PR correcting these comments (I haven't done in the same PR as it seems two different problems).

Additionally, I worked out that RENDER_DEPTH seems actually for the z pre-pass and shadow maps. If so perhaps a better define would be RENDER_DEPTH_ONLY, because depth is also rendered during normal passes, so the define is confusing.

Also, //ubershader-runtime seems to be put on every closing #endif, and I have no idea why, perhaps I can figure out from the ubershader PR. 😁

[lawnjelly]

Moved this because it turns out RENDER_DEPTH is actually depth only pass, and we don't want to write to color in depth only pass.

(TURNS OUT - unless we are using RGBA shadows 😀 ).

[lawnjelly]

This comment may be incorrect, should be // not USE_MULTIPLE_RENDER_TARGETS...

[lawnjelly]

This comment seems incorrect, should be // not RENDER_DEPTH_ONLY.

[clayjohn]

This looks good. The problem comes from here where we define diffuse_buffer to be frag_color:

godot/drivers/gles3/shaders/scene.glsl

Line 927 in bb342cb

#define diffuse_buffer frag_color

So the main output of the shader ultimately writes to frag_color (our intermediate color value). Accordingly, we need to assign it to frag_color_final outside the branch

[lawnjelly]

Fixed one last tweak neither of us noticed - the GLES2 change would have broken RGBA shadows because it never wrote to glFragColor in the very last section 😊 .

Have fixed this.

[lawnjelly]

Thanks!