### 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 (openxr_api) {
		XrEnvironmentBlendMode oxr_blend_mode;
		switch (mode) {
			case XR_ENV_BLEND_MODE_OPAQUE:
				oxr_blend_mode = XR_ENVIRONMENT_BLEND_MODE_OPAQUE;
				break;
			case XR_ENV_BLEND_MODE_ADDITIVE:
				oxr_blend_mode = XR_ENVIRONMENT_BLEND_MODE_ADDITIVE;
				break;
			case XR_ENV_BLEND_MODE_ALPHA_BLEND:
				oxr_blend_mode = XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND;
				break;
			default:
				WARN_PRINT("Unknown blend mode requested: " + String::num_int64(int64_t(mode)));
				oxr_blend_mode = XR_ENVIRONMENT_BLEND_MODE_OPAQUE;
		}
		return openxr_api->set_environment_blend_mode(oxr_blend_mode);
	}
	return false;
}
void OpenXRInterface::on_state_ready() {
	emit_signal(SNAME("session_begun"));
}
void OpenXRInterface::on_state_visible() {
	emit_signal(SNAME("session_visible"));
}
void OpenXRInterface::on_state_focused() {