### 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
GLOBAL_DEF_RST("rendering/gl_compatibility/nvidia_disable_threaded_optimization", true);
	}
	
#ifdef VULKAN_ENABLED
	renderer_hints = "forward_plus,mobile";
	default_renderer_mobile = "mobile";
#endif
	
#ifdef GLES3_ENABLED
	if (!renderer_hints.is_empty()) {
		renderer_hints += ",";
	}
	renderer_hints += "gl_compatibility";
	if (default_renderer_mobile.is_empty()) {
		default_renderer_mobile = "gl_compatibility";
	}
	
	if (rendering_driver.is_empty() && rendering_method.is_empty() && project_manager) {
		rendering_driver = "opengl3";
		rendering_method = "gl_compatibility";
		default_renderer_mobile = "gl_compatibility";
	}
#endif
	if (renderer_hints.is_empty()) {
		ERR_PRINT("No renderers available.");
	}
	if (!rendering_method.is_empty()) {
		if (rendering_method != "forward_plus" &&
				rendering_method != "mobile" &&
				rendering_method != "gl_compatibility") {