### 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
} else {
				OS::get_singleton()->print("Missing write-movie argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--disable-vsync") {
			disable_vsync = true;
		} else if (I->get() == "--print-fps") {
			print_fps = true;
		} else if (I->get() == "--profile-gpu") {
			profile_gpu = true;
		} else if (I->get() == "--disable-crash-handler") {
			OS::get_singleton()->disable_crash_handler();
		} else if (I->get() == "--skip-breakpoints") {
			skip_breakpoints = true;
		} else if (I->get() == "--xr-mode") {
			if (I->next()) {
				String xr_mode = I->next()->get().to_lower();
				N = I->next()->next();
				if (xr_mode == "default") {
					XRServer::set_xr_mode(XRServer::XRMODE_DEFAULT);
				} else if (xr_mode == "off") {
					XRServer::set_xr_mode(XRServer::XRMODE_OFF);
				} else if (xr_mode == "on") {
					XRServer::set_xr_mode(XRServer::XRMODE_ON);
				} else {
					OS::get_singleton()->print("Unknown --xr-mode argument \"%s\", aborting.\n", xr_mode.ascii().get_data());
					goto error;
				}
			} else {
				OS::get_singleton()->print("Missing --xr-mode argument, aborting.\n");