OpenXR: change bindings to 'flatten' source paths

[BastiaanOlij]

Our OpenXR action map currently stores multiple input/output paths (now called source paths) per action in an interaction profile.

This causes an issue with upcoming binding modifiers (see #97140).

This PR will "flatten" this structure so one binding records a single binding between an action and a source path.
If multiple paths should be bound, multiple binding entries are needed.

This is technically a breaking change in that the action map will be altered in such a way that it can't be loaded by older versions of Godot, but it is required to implement future modifiers.

Contributed by Khronos Group through the Godot Integration Project

[dsnopek]

Overall, this restructuring of the action map data makes sense to me!

This is also a breaking change with regard to code that programmatically creates an action map with multiple paths per binding. However, I think it's pretty rare to programmatically create an action map, and even so, it would only affect bindings with multiple paths, which even further reduces the affected code. I personally think this is acceptable.

In 99.9% of cases, developers will create action maps in the editor, and this should correctly load and convert existing action maps (although, I haven't tested it - only reviewed the code).

TL;DR LGTM :-)

Only a handful of minor notes.

[BastiaanOlij]

I'm also renaming source_path to binding_path after an internal discussion that source path misleads developers in thinking the bindings are absolute. This is a common misperception around the action map where bindings are suggestions that the XR runtime can override either due to user preferences or due to other types of hardware being used.

[dsnopek]

Overall, this looks great to me!

I personally think we could go a little further with removing the code for old action maps in deprecated=no builds, but that shouldn't necessarily hold this up.

[BastiaanOlij]

Everything should be cleared up now, should be ready to be merged.

[Repiteo]

Thanks!