### 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
cmdline_tool = true;
			main_args.push_back(I->get());
#ifndef DISABLE_DEPRECATED
		} else if (I->get() == "--export") { 
			OS::get_singleton()->print("The Godot 3 --export option was changed to more explicit --export-release / --export-debug / --export-pack options.\nSee the --help output for details.\n");
			goto error;
		} else if (I->get() == "--convert-3to4") {
			
			cmdline_tool = true;
			main_args.push_back(I->get());
			if (I->next() && !I->next()->get().begins_with("-")) {
				if (itos(I->next()->get().to_int()) == I->next()->get()) {
					converter_max_kb_file = I->next()->get().to_int();
				}
				if (I->next()->next() && !I->next()->next()->get().begins_with("-")) {
					if (itos(I->next()->next()->get().to_int()) == I->next()->next()->get()) {
						converter_max_line_length = I->next()->next()->get().to_int();
					}
				}
			}
		} else if (I->get() == "--validate-conversion-3to4") {
			
			cmdline_tool = true;
			main_args.push_back(I->get());
			if (I->next() && !I->next()->get().begins_with("-")) {
				if (itos(I->next()->get().to_int()) == I->next()->get()) {
					converter_max_kb_file = I->next()->get().to_int();
				}
				if (I->next()->next() && !I->next()->next()->get().begins_with("-")) {
					if (itos(I->next()->next()->get().to_int()) == I->next()->next()->get()) {