### 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
Engine::get_singleton()->set_max_fps(max_fps);
	}
	if (frame_delay == 0) {
		frame_delay = GLOBAL_DEF(PropertyInfo(Variant::INT, "application/run/frame_delay_msec", PROPERTY_HINT_RANGE, "0,100,1,or_greater"), 0);
	}
	if (audio_output_latency >= 1) {
		Engine::get_singleton()->set_audio_output_latency(audio_output_latency);
	}
	OS::get_singleton()->set_low_processor_usage_mode(GLOBAL_DEF("application/run/low_processor_mode", false));
	OS::get_singleton()->set_low_processor_usage_mode_sleep_usec(
			GLOBAL_DEF(PropertyInfo(Variant::INT, "application/run/low_processor_mode_sleep_usec", PROPERTY_HINT_RANGE, "0,33200,1,or_greater"), 6900)); 
	GLOBAL_DEF("application/run/delta_smoothing", true);
	if (!delta_smoothing_override) {
		OS::get_singleton()->set_delta_smoothing(GLOBAL_GET("application/run/delta_smoothing"));
	}
	GLOBAL_DEF("display/window/ios/allow_high_refresh_rate", true);
	GLOBAL_DEF("display/window/ios/hide_home_indicator", true);
	GLOBAL_DEF("display/window/ios/hide_status_bar", true);
	GLOBAL_DEF("display/window/ios/suppress_ui_gesture", true);
	
	GLOBAL_DEF_RST_BASIC("xr/openxr/enabled", false);
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::STRING, "xr/openxr/default_action_map", PROPERTY_HINT_FILE, "*.tres"), "res://openxr_action_map.tres");
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::INT, "xr/openxr/form_factor", PROPERTY_HINT_ENUM, "Head Mounted,Handheld"), "0");
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::INT, "xr/openxr/view_configuration", PROPERTY_HINT_ENUM, "Mono,Stereo"), "1"); 
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::INT, "xr/openxr/reference_space", PROPERTY_HINT_ENUM, "Local,Stage"), "1");
	GLOBAL_DEF_BASIC("xr/openxr/submit_depth_buffer", false);
	GLOBAL_DEF_BASIC("xr/openxr/startup_alert", true);
#ifdef TOOLS_ENABLED