### 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
cs_aspect = Window::CONTENT_SCALE_ASPECT_KEEP_HEIGHT;
			} else if (stretch_aspect == "expand") {
				cs_aspect = Window::CONTENT_SCALE_ASPECT_EXPAND;
			}
			Window::ContentScaleStretch cs_stretch = Window::CONTENT_SCALE_STRETCH_FRACTIONAL;
			if (stretch_scale_mode == "integer") {
				cs_stretch = Window::CONTENT_SCALE_STRETCH_INTEGER;
			}
			sml->get_root()->set_content_scale_mode(cs_sm);
			sml->get_root()->set_content_scale_aspect(cs_aspect);
			sml->get_root()->set_content_scale_stretch(cs_stretch);
			sml->get_root()->set_content_scale_size(stretch_size);
			sml->get_root()->set_content_scale_factor(stretch_scale);
			sml->set_auto_accept_quit(GLOBAL_GET("application/config/auto_accept_quit"));
			sml->set_quit_on_go_back(GLOBAL_GET("application/config/quit_on_go_back"));
			String appname = GLOBAL_GET("application/config/name");
			appname = TranslationServer::get_singleton()->translate(appname);
#ifdef DEBUG_ENABLED
			/
			/
			/
			DisplayServer::get_singleton()->window_set_title(vformat("%s (DEBUG)", appname));
#else
			DisplayServer::get_singleton()->window_set_title(appname);
#endif
			bool snap_controls = GLOBAL_GET("gui/common/snap_controls_to_pixels");
			sml->get_root()->set_snap_controls_to_pixels(snap_controls);
			bool font_oversampling = GLOBAL_GET("gui/fonts/dynamic_fonts/use_oversampling");
			sml->get_root()->set_use_font_oversampling(font_oversampling);
			int texture_filter = GLOBAL_GET("rendering/textures/canvas_textures/default_texture_filter");