### 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
Size2 target_size = get_render_target_size();
	real_t aspect = target_size.x / target_size.y; 
	Size2 vrs_size = Size2(round(0.5 + target_size.x / texel_width), round(0.5 + target_size.y / texel_height));
	real_t radius = vrs_size.length() * 0.5;
	Size2 vrs_sizei = vrs_size;
	if (vrs.size != vrs_sizei) {
		const uint8_t densities[] = {
			0, 
			1, 
			
			
			
			5, 
			6, 
			
			10, 
		};
		
		if (vrs.vrs_texture.is_valid()) {
			RS::get_singleton()->free(vrs.vrs_texture);
			vrs.vrs_texture = RID();
		}
		
		Vector<Ref<Image>> images;
		vrs.size = vrs_sizei;
		for (int i = 0; i < view_count && i < 2; i++) {
			PackedByteArray data;
			data.resize(vrs_sizei.x * vrs_sizei.y);
			uint8_t *data_ptr = data.ptrw();