### 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 remote debug host address, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--editor-pid") { 
			if (I->next()) {
				editor_pid = I->next()->get().to_int();
				N = I->next()->next();
			} else {
				OS::get_singleton()->print("Missing editor PID argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--disable-render-loop") {
			disable_render_loop = true;
		} else if (I->get() == "--fixed-fps") {
			if (I->next()) {
				fixed_fps = I->next()->get().to_int();
				N = I->next()->next();
			} else {
				OS::get_singleton()->print("Missing fixed-fps argument, aborting.\n");
				goto error;
			}
		} else if (I->get() == "--write-movie") {
			if (I->next()) {
				Engine::get_singleton()->set_write_movie_path(I->next()->get());
				N = I->next()->next();
				if (fixed_fps == -1) {
					fixed_fps = 60;
				}
				OS::get_singleton()->_writing_movie = true;