### 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
Projection cm = get_projection_for_view(i, aspect, 0.1, 1000.0);
			Vector3 center = cm.xform(Vector3(0.0, 0.0, 999.0));
			Vector2i view_center;
			view_center.x = int(vrs_size.x * (center.x + 1.0) * 0.5);
			view_center.y = int(vrs_size.y * (center.y + 1.0) * 0.5);
			int d = 0;
			for (int y = 0; y < vrs_sizei.y; y++) {
				for (int x = 0; x < vrs_sizei.x; x++) {
					Vector2 offset = Vector2(x - view_center.x, y - view_center.y);
					offset.y *= aspect;
					real_t distance = offset.length();
					int idx = round(5.0 * distance / radius);
					if (idx > 4) {
						idx = 4;
					}
					uint8_t density = densities[idx];
					data_ptr[d++] = density;
				}
			}
			images.push_back(Image::create_from_data(vrs_sizei.x, vrs_sizei.y, false, Image::FORMAT_R8, data));
		}
		if (images.size() == 1) {
			vrs.vrs_texture = RS::get_singleton()->texture_2d_create(images[0]);
		} else {
			vrs.vrs_texture = RS::get_singleton()->texture_2d_layered_create(images, RS::TEXTURE_LAYERED_2D_ARRAY);
		}
	}
	return vrs.vrs_texture;
}
RID XRInterface::get_color_texture() {