Support making hybrid apps

[dsnopek]

This uses Godot 4.4-dev5 and must be used with godot_openxr_vendors PR GodotVR/godot_openxr_vendors#231

You use an export setting to make a hybrid app:

You can choose Start As Immersive or Start As Panel, but when run from the Godot editor, it'll always start in immersive mode because the editor directly names the activity it launches (it's hard-coded) and the normal Godot game activity is always the immersive one (see notes below).

There is a decent GDScript API:

# Returns true or false for if this is a hybrid app
MetaHybridApp.is_hybrid_app()

# Returns an enum with values: NONE, IMMERSIVE or PANEL
MetaHybridApp.get_mode()

# Allows switching modes and passing a string as "launch data"
MetaHybridApp.switch_mode(MetaHybridApp.HybridMode.PANEL, '{"info": "the info!"}')

# And this function lets you get the "launch data"
MetaHybridApp.get_launch_data()

The demo shows a simple way to use these.

[m4gr3d]

This should be under a if (hybrid_type != HYBRID_TYPE_DISABLED) check so we don't override this option when hybrid is disabled; e.g: if the user sets this to false we would override it back to true with the current logic if hybrid is disabled.

[m4gr3d]

Note that this manifest entry constrains the panel resizing so as to maintain the panel's aspect ratio; I'm fine with this being the default behavior, just to ensure this is intended behavior since it's hardcoded.

[m4gr3d]

We can include this directly in the manifest for the meta vendor plugin once the PR is moved. In general if the manifest entry is not modifiable, then it should be included directly in the vendor specific manifest file.

[dsnopek]

As discussed, I've moved this to PR GodotVR/godot_openxr_vendors#245 against "godot_openxr_vendors", and addressed the review here as well as the other things we discussed.

Closing as superseded.