### 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
Error exit_code = ERR_INVALID_PARAMETER;
	I = args.front();
	while (I) {
#ifdef MACOS_ENABLED
		
		
		if (I->get().begins_with("-psn_")) {
			I = I->next();
			continue;
		}
#endif
		List<String>::Element *N = I->next();
#ifdef TOOLS_ENABLED
		if (I->get() == "--debug" ||
				I->get() == "--verbose" ||
				I->get() == "--disable-crash-handler") {
			forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->get());
			forwardable_cli_arguments[CLI_SCOPE_PROJECT].push_back(I->get());
		}
		if (I->get() == "--single-window") {
			forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->get());
		}
		if (I->get() == "--audio-driver" ||
				I->get() == "--display-driver" ||
				I->get() == "--rendering-method" ||
				I->get() == "--rendering-driver") {
			if (I->next()) {
				forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->get());
				forwardable_cli_arguments[CLI_SCOPE_TOOL].push_back(I->next()->get());