Fix cross-platform configuration of rendering driver settings (narrower approach)

[akien-mga]

Simpler alternative to #103026 which avoids breaking compatibility.

Instead of introducing a new auto default value, we ensure that all supported drivers are registered regardless of the editor's host platform, and that the defaults are the intended ones.

This solves the following issues:

• macOS exports are meant to default to Metal in 4.4, but they would default to Vulkan if exported from Linux, Windows, or Android editors.
• Windows exports couldn't be made with Direct3D 12 from Linux, macOS, or Android editors, as the option couldn't be selected outside Windows.

Unlike #103026, it doesn't solve the issue of not always saving the rendering drivers to project.godot, but now the defaults are at least consistent between editor platforms. So that issue is a lot less problematic, or maybe not even an issue anymore, but may be considered working as intended (so we can keep changing defaults if we want and have projects auto-upgrade).

• Reverts #103026.
• Fixes #103156.
• Partially supersedes #103144. Some of the changes there are still worth doing, I can add them to this PR too, or #103144 could be rebased after this is merged.

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Before (Linux editor):

After (Linux editor):