Add a project setting for the Shader editor to warn/error as if using the gl_compatibility renderer

[clayjohn]

Describe the project you are working on

The Godot Engine

Describe the problem or limitation you are having in your project

We flag shader functions as high-end (only usable from RD backends) or low end (can be used in any backend). If a user writes a shader and includes a high-end function while using the gl_compatibility renderer, the shader editor will provide an error and explain that the function is not supported.

The issue arises for users who develop using an RD-based backend (Forward+ or Mobile) and then export using gl_compatibility (for example if the export to web). The error in that case will arise in the exported project but not in the editor.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

I suggest that we add a project setting like emulate_low_end that make the shader compiler provide errors as if it were using a low end renderer. That way users can author their shaders in a way that will be supported on all devices regardless of what renderer they have selected while authoring.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

We could implement it by changing this line:
https://github.com/godotengine/godot/blob/44c0bfc94d81e758b39a8ee43df5915d64200ed6/servers/rendering/shader_language.cpp#L3093
to:

if (RenderingServer::get_singleton()->is_low_end() || emulate_low_end ) {

emulate_low_end would be a variable that is set based on the project setting.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No

Is there a reason why this should be core and not an add-on in the asset library?

It requires core changes

[lostminds]

I think this could be helpful for cases like those discussed in godotengine/godot#72016 . However, emulate_low_end sounds to me like it would not just show warnings about unsupported features. Perhaps it could be called something more descriptive like shader_editor_show_low_end_compatibility_warnings if that's what it will actually do?

Even better would be if these warnings could also include hints on how to make fallbacks (like making an instance uniform a constant) if possible for the shader in gl_compatibility mode to make it compile there while still keeping the advanced feature in use when supported.

[Zireael07]

show_low_end_warnings would imho be the compromise name that would not be too wordy (unlike what @lostminds proposed) and better reflect what happens in editor.