### 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 (display_server->has_feature(DisplayServer::FEATURE_ORIENTATION)) {
		display_server->screen_set_orientation(window_orientation);
	}
	if (GLOBAL_GET("debug/settings/stdout/print_fps") || print_fps) {
		
		switch (window_vsync_mode) {
			case DisplayServer::VSyncMode::VSYNC_DISABLED:
				print_line("Requested V-Sync mode: Disabled");
				break;
			case DisplayServer::VSyncMode::VSYNC_ENABLED:
				print_line("Requested V-Sync mode: Enabled - FPS will likely be capped to the monitor refresh rate.");
				break;
			case DisplayServer::VSyncMode::VSYNC_ADAPTIVE:
				print_line("Requested V-Sync mode: Adaptive");
				break;
			case DisplayServer::VSyncMode::VSYNC_MAILBOX:
				print_line("Requested V-Sync mode: Mailbox");
				break;
		}
	}
	if (OS::get_singleton()->_render_thread_mode == OS::RENDER_SEPARATE_THREAD) {
		WARN_PRINT("The Multi-Threaded rendering thread model is experimental, and has known issues which can lead to project crashes. Use the Single-Safe option in the project settings instead.");
	}
	
	{
		GLOBAL_DEF_RST_NOVAL("input_devices/pen_tablet/driver", "");
		GLOBAL_DEF_RST_NOVAL(PropertyInfo(Variant::STRING, "input_devices/pen_tablet/driver.windows", PROPERTY_HINT_ENUM, "wintab,winink"), "");
	}
	if (tablet_driver.is_empty()) { 
		tablet_driver = GLOBAL_GET("input_devices/pen_tablet/driver");