### 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
ShaderCompiler::GeneratedCode gen_code;
	int blend_mode = BLEND_MODE_MIX;
	int depth_testi = DEPTH_TEST_ENABLED;
	int alpha_antialiasing_mode = ALPHA_ANTIALIASING_OFF;
	int cull = CULL_BACK;
	uses_point_size = false;
	uses_alpha = false;
	uses_alpha_clip = false;
	uses_alpha_antialiasing = false;
	uses_blend_alpha = false;
	uses_depth_prepass_alpha = false;
	uses_discard = false;
	uses_roughness = false;
	uses_normal = false;
	bool wireframe = false;
	unshaded = false;
	uses_vertex = false;
	uses_sss = false;
	uses_transmittance = false;
	uses_time = false;
	writes_modelview_or_projection = false;
	uses_world_coordinates = false;
	uses_particle_trails = false;
	int depth_drawi = DEPTH_DRAW_OPAQUE;
	ShaderCompiler::IdentifierActions actions;
	actions.entry_point_stages["vertex"] = ShaderCompiler::STAGE_VERTEX;
	actions.entry_point_stages["fragment"] = ShaderCompiler::STAGE_FRAGMENT;
	actions.entry_point_stages["light"] = ShaderCompiler::STAGE_FRAGMENT;
	actions.render_mode_values["blend_add"] = Pair<int *, int>(&blend_mode, BLEND_MODE_ADD);
	actions.render_mode_values["blend_mix"] = Pair<int *, int>(&blend_mode, BLEND_MODE_MIX);