### Godot version

4.0.2 stable

### System information

issue tested on win10 with ryzen 7 and rx 6600, win11 with ryzen 7 and gtx 3050

### Issue description

When creating a screen reading post processing shader, the normal buffer functions correctly in the editor, but does not seem to be functioning as expected in debug, producing only a solid colour when run. I am unsure if this behaviour is intended, but if so, I feel it should be properly documented.

Running correctly in editor:
![image](https://user-images.githubusercontent.com/71034402/230672620-fc5e4baf-c12e-4755-b7a2-206087459307.png)

Solid colour in debug:
![image](https://user-images.githubusercontent.com/71034402/230672489-d9c29755-c2d3-4e72-9ea0-355148e916fe.png)


### Steps to reproduce

*this shader should be attached to a MeshInstance3D quadmesh*
minimal shader to reproduce:

```glsl
shader_type spatial;
render_mode unshaded, cull_disabled;

uniform sampler2D NORMAL_TEXTURE : hint_normal_roughness_texture, filter_linear_mipmap;

void vertex(){
	VERTEX *= 2.0;
	POSITION = vec4(VERTEX, 1.0);
}

void fragment(){
	ALBEDO = texture(NORMAL_TEXTURE, SCREEN_UV).rgb;
}
```

### Minimal reproduction project

N/A
if ((p_pass_mode == PASS_MODE_COLOR && !(p_color_pass_flags & COLOR_PASS_FLAG_TRANSPARENT)) && !(surf->flags & GeometryInstanceSurfaceDataCache::FLAG_PASS_OPAQUE)) {
			continue; 
		}
		if (surf->owner->instance_count == 0) {
			continue;
		}
		push_constant.base_index = i + p_params->element_offset;
		RID material_uniform_set;
		SceneShaderForwardClustered::ShaderData *shader;
		void *mesh_surface;
		if (shadow_pass || p_pass_mode == PASS_MODE_DEPTH) { 
			material_uniform_set = surf->material_uniform_set_shadow;
			shader = surf->shader_shadow;
			mesh_surface = surf->surface_shadow;
		} else {
#ifdef DEBUG_ENABLED
			if (unlikely(get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_LIGHTING)) {
				material_uniform_set = scene_shader.default_material_uniform_set;
				shader = scene_shader.default_material_shader_ptr;
			} else if (unlikely(get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_OVERDRAW)) {
				material_uniform_set = scene_shader.overdraw_material_uniform_set;
				shader = scene_shader.overdraw_material_shader_ptr;
			} else if (unlikely(get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_PSSM_SPLITS)) {
				material_uniform_set = scene_shader.debug_shadow_splits_material_uniform_set;
				shader = scene_shader.debug_shadow_splits_material_shader_ptr;
			} else {
#endif
				material_uniform_set = surf->material_uniform_set;
				shader = surf->shader;
				surf->material->set_as_used();