### Godot version

4.0.2.stable

### System information

Windows 11, Vulkan, RTX2070 Super

### Issue description

Reading from `VIEWPORT_SIZE` in the `light()` function causes spatial shader to break and renders as default white.

### Steps to reproduce

![image](https://user-images.githubusercontent.com/20043270/236914345-ccd12e83-4431-4a53-b67b-028c8ef5719b.png)

when line 51 is present the fails to compile with the following message in the output log but no compilation errors.

```
servers/rendering/renderer_rd/forward_clustered/scene_shader_forward_clustered.cpp:171 - Condition "!shader_singleton->shader.version_is_valid(version)" is true.
  drivers/vulkan/rendering_device_vulkan.cpp:5361 - Condition "!shader" is true. Returning: RID()
  drivers/vulkan/rendering_device_vulkan.cpp:5835 - Condition "!us" is true.
```

commenting out line 51 solves this issue.

[The documentation](https://docs.godotengine.org/en/stable/tutorials/shaders/shader_reference/spatial_shader.html#light-built-ins) states that `VIEWPORT_SIZE` should be available to the `light()` function.

### Minimal reproduction project

N/A
return (uses_fragment_time && uses_discard) || (uses_vertex_time && uses_vertex);
}
bool SceneShaderForwardMobile::ShaderData::casts_shadows() const {
	bool has_read_screen_alpha = uses_screen_texture || uses_depth_texture || uses_normal_texture;
	bool has_base_alpha = (uses_alpha && (!uses_alpha_clip || uses_alpha_antialiasing)) || has_read_screen_alpha;
	bool has_alpha = has_base_alpha || uses_blend_alpha;
	return !has_alpha || (uses_depth_prepass_alpha && !(depth_draw == DEPTH_DRAW_DISABLED || depth_test == DEPTH_TEST_DISABLED));
}
RS::ShaderNativeSourceCode SceneShaderForwardMobile::ShaderData::get_native_source_code() const {
	SceneShaderForwardMobile *shader_singleton = (SceneShaderForwardMobile *)SceneShaderForwardMobile::singleton;
	return shader_singleton->shader.version_get_native_source_code(version);
}
SceneShaderForwardMobile::ShaderData::ShaderData() :
		shader_list_element(this) {
}
SceneShaderForwardMobile::ShaderData::~ShaderData() {
	SceneShaderForwardMobile *shader_singleton = (SceneShaderForwardMobile *)SceneShaderForwardMobile::singleton;
	ERR_FAIL_COND(!shader_singleton);
	
	if (version.is_valid()) {
		shader_singleton->shader.version_free(version);
	}
}
RendererRD::MaterialStorage::ShaderData *SceneShaderForwardMobile::_create_shader_func() {