Fix stray pixel artifacts when BaseMaterial3D Deep Parallax is enabled

[Calinou]

• Related to Rework BaseMaterial3D deep parallax implementation and add new features #97646.

Mipmaps for the heightmap seem to cause issues with the parallax occlusion mapping algorithm used. This is most noticeable with steep height changes where surrounding pixels will occasionally appear depending on the view angle. This occurs regardless of the Min Layers and Max Layers values.

Note that there may be a better solution to this, such as sampling the heightmap texture differently (e.g. textureGrad()). This could result in better performance as mipmaps allow the GPU to sample a lower-resolution texture in the distance – however, for this to be beneficial, the different texture sampling method must not be more expensive than disabling mipmaps.

I found this issue (and the way to fix it) using https://github.com/Calinou/godot-rendering-tests' texture sampling test.

Edit: Xtarsia mentioned on the Godot VFX Discord that this could resolved in a more optimized way in Forward+/Mobile:

textureLod() for the heightmap read, rather than disabling mipmaps entirely?
1 call to textureQueryLod() before the loop should be sufficient. This is likley to be faster than no mipmaps at all?

Can fall back to textureLod(height_sampler, uv, 0.0) for Compatibility

Testing project: https://github.com/Calinou/godot-parallax-test-4.0

Preview

Before

parallax_before.mp4

After

parallax_after.mp4