### 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
print_line("OpenXR: Couldn't locate views [", get_error_string(result), "]");
		return;
	}
	bool pose_valid = true;
	for (uint64_t i = 0; i < view_count_output; i++) {
		if ((view_state.viewStateFlags & XR_VIEW_STATE_ORIENTATION_VALID_BIT) == 0 ||
				(view_state.viewStateFlags & XR_VIEW_STATE_POSITION_VALID_BIT) == 0) {
			pose_valid = false;
		}
	}
	if (view_pose_valid != pose_valid) {
		view_pose_valid = pose_valid;
		if (!view_pose_valid) {
			print_verbose("OpenXR View pose became invalid");
		} else {
			print_verbose("OpenXR View pose became valid");
		}
	}
	
	XrFrameBeginInfo frame_begin_info = {
		XR_TYPE_FRAME_BEGIN_INFO, 
		nullptr 
	};
	result = xrBeginFrame(session, &frame_begin_info);
	if (XR_FAILED(result)) {
		print_line("OpenXR: failed to being frame [", get_error_string(result), "]");
		return;
	}
}
bool OpenXRAPI::pre_draw_viewport(RID p_render_target) {