### Godot version

4.1

### System information

Windows 10 - Godot 4.1.1 - Compatibility

### Issue description

When trying to use the Varjo XR3 headset (in OpenXR mode) with Godot 4 (with OpenXR and passthrough activated), the passthrough does not work, i.e. the scene is rendered on a black background instead of the camera stream.

It seems to me that are two problems in the openxr module, more precisely in the openx_api.cpp file (fixing them solves  the issue) : 

1. The XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT  is only set when the number of layers is > 1, which is fine for the Meta Quest headset but does not work for the Varjo since it only requires one layer. 
2. The environment blend mode XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND  is only really set when the XRFbPassthrough extension is present, which is not the case with the Varjo runtime. 

I am not sure how to fix this without causing issues for other headsets.

1. Would it be an issue to always activate the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT ? 
2. Could blend modes be activated even if they are not supported by an extension ? 





### Steps to reproduce

- Create a Godot 4 project with OpenXR activated in Compatibility mode
- Add the code to activate OpenXr and to enable passthrough : https://docs.godotengine.org/en/stable/tutorials/xr/openxr_passthrough.html#passthrough-extension
- Start the Varjo Base software and activate OpenXR
- Run the Godot project

### Minimal reproduction project

N/A
case XR_TYPE_EVENT_DATA_INSTANCE_LOSS_PENDING: {
				XrEventDataInstanceLossPending *event = (XrEventDataInstanceLossPending *)&runtimeEvent;
				
				
				print_verbose(String("OpenXR EVENT: instance loss pending at ") + itos(event->lossTime));
				return false;
			} break;
			case XR_TYPE_EVENT_DATA_SESSION_STATE_CHANGED: {
				XrEventDataSessionStateChanged *event = (XrEventDataSessionStateChanged *)&runtimeEvent;
				session_state = event->state;
				if (session_state >= XR_SESSION_STATE_MAX_ENUM) {
					print_verbose(String("OpenXR EVENT: session state changed to UNKNOWN - ") + itos(session_state));
				} else {
					print_verbose(String("OpenXR EVENT: session state changed to ") + OpenXRUtil::get_session_state_name(session_state));
					switch (session_state) {
						case XR_SESSION_STATE_IDLE:
							on_state_idle();
							break;
						case XR_SESSION_STATE_READY:
							on_state_ready();
							break;
						case XR_SESSION_STATE_SYNCHRONIZED:
							on_state_synchronized();
							break;
						case XR_SESSION_STATE_VISIBLE:
							on_state_visible();
							break;
						case XR_SESSION_STATE_FOCUSED:
							on_state_focused();
							break;