[4.3] Add CLIP_SPACE_FAR built-in to spatial shader

[aaronfranke]

Manual backport of PR #95057 to the Godot 4.3 branch because there were conflicts. I tested that this works.

I resolved conflicts, but otherwise the code is entirely from @Chaosus's original PR.

[paddy-exe]

Is there a specific reason why this particular feature should be backported to 4.3?
This goes against semantic versioning specs.

[dsnopek]

I think backporting this could make sense.

Shaders that write to POSITION need to know the CLIP_SPACE_FAR, which is different between compatibility and the RD renderers. Having this in Godot 4.3 allows putting shaders on the Godot asset library that are compatible with more renderers and versions.

That said, as we keep moving forward, and Godot 4.3 gets older and older, I think this makes less sense.

[aaronfranke]

@paddy-exe I have a project which tries to target multiple Godot versions and renderers, and makes use of this constant. The lack of this feature is the biggest problem inhibiting 4.3 support. I would greatly appreciate having this backported.

In my case it's an engine module, so just having this in the 4.3 branch is sufficient, even if it's not in a patch release yet.

[paddy-exe]

@dsnopek @aaronfranke Be that as it may, I believe this backport would set a bad precedent for backporting features to a previous release branch. The same logic could probably be used by several other plugin/extension developers and their cases (both for past and future versions).

I will put this up for discussion at the next rendering meeting.

[paddy-exe]

After discussing this in the rendering meeting, the team has concluded that the decision if this should be merged belongs more to the release management team. Moreover, it declared the PR helpful for shader authors and a very safe change.