Add "NO_SHADOWS" shader version to Mobile backend

[darksylinc]

Before this patch, if the dev disabled shadows on all lights Godot would avoid the shadow casting pass but it would still use a pixel shader that tried to sample an empty shadow map (i.e. it's always shadow = 1.0).

After this patch, we compile a variant that has samples no shadows.

The shader already had support for this (#define SHADOWS_DISABLED) but it was not being used for this purpose. It was only used if the material had specifically enabled a setting to not receive shadows.

This greatly improves performance on low end GPUs such as Adreno 505.

Note that low end mobile has multiple bottlenecks at the same time (we're everything-bound 🤣 ) and this is the first PR in a series of PRs to improve mobile performance.

[BastiaanOlij]

I think we need to further improve on this, though possibly this can be finetuned with specialisation constants.

The most common scenario I can think of is where we only have shadows on directional lights and not on any omni/spot lights.

Or like a horror game where only a flashlight casts shadows and all other lights either have no shadows or are baked.

[Calinou]

• Related to Add a global enable/disable shadows godot-proposals#326.
  • Note that despite what's been said in the proposal, you can disable positional shadows globally by setting the shadow atlas size to 0 in the Viewport. The Project Settings won't allow you to do this due to its property hint though, but we can modify it to allow this.
  • There is no way to globally disable directional shadows though, but doing so affects visuals a lot more than disabling shadows for positional lights in most scenes. There are games which support this (e.g. Fortnite's Performance mode). It looks bad, but it can be the difference between an unplayable game and a playable game on a low-end device.