### 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
if (rendering_method.is_empty()) {
			if (rendering_driver == "opengl3") {
				rendering_method = "gl_compatibility";
			} else {
				rendering_method = "forward_plus";
			}
		}
		
		bool valid_combination = false;
		Vector<String> available_drivers;
#ifdef VULKAN_ENABLED
		if (rendering_method == "forward_plus" || rendering_method == "mobile") {
			available_drivers.push_back("vulkan");
		}
#endif
#ifdef GLES3_ENABLED
		if (rendering_method == "gl_compatibility") {
			available_drivers.push_back("opengl3");
		}
#endif
		if (available_drivers.is_empty()) {
			OS::get_singleton()->print("Unknown renderer name '%s', aborting.\n", rendering_method.utf8().get_data());
			goto error;
		}
		for (int i = 0; i < available_drivers.size(); i++) {
			if (rendering_driver == available_drivers[i]) {
				valid_combination = true;
				break;
			}