### 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 (boot_logo.is_valid()) {
			RenderingServer::get_singleton()->set_boot_image(boot_logo, boot_bg_color, boot_logo_scale,
					boot_logo_filter);
		} else {
#ifndef NO_DEFAULT_BOOT_LOGO
			MAIN_PRINT("Main: Create bootsplash");
#if defined(TOOLS_ENABLED) && !defined(NO_EDITOR_SPLASH)
			Ref<Image> splash = (editor || project_manager) ? memnew(Image(boot_splash_editor_png)) : memnew(Image(boot_splash_png));
#else
			Ref<Image> splash = memnew(Image(boot_splash_png));
#endif
			MAIN_PRINT("Main: ClearColor");
			RenderingServer::get_singleton()->set_default_clear_color(boot_bg_color);
			MAIN_PRINT("Main: Image");
			RenderingServer::get_singleton()->set_boot_image(splash, boot_bg_color, false);
#endif
		}
#if defined(TOOLS_ENABLED) && defined(MACOS_ENABLED)
		if (OS::get_singleton()->get_bundle_icon_path().is_empty()) {
			Ref<Image> icon = memnew(Image(app_icon_png));
			DisplayServer::get_singleton()->set_icon(icon);
		}
#endif
	}
	DisplayServer::set_early_window_clear_color_override(false);
	MAIN_PRINT("Main: DCC");
	RenderingServer::get_singleton()->set_default_clear_color(
			GLOBAL_GET("rendering/environment/defaults/default_clear_color"));
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::STRING, "application/config/icon", PROPERTY_HINT_FILE, "*.png,*.webp,*.svg"), String());
	GLOBAL_DEF(PropertyInfo(Variant::STRING, "application/config/macos_native_icon", PROPERTY_HINT_FILE, "*.icns"), String());