[3.x] Allow Material3D Depth in the GLES2 backend

[Calinou]

Follow-up to https://github.com/godotengine/godot/pull/50295.

On GLES2, the shader now uses a fixed for loop instead of a dynamic while loop as while loops aren't supported on all platforms in GLES2.

I've tested both simple (non-deep) parallax and deep parallax on desktop, HTML5 and Android. They all work as expected now.

Inspired by three.js: https://github.com/mrdoob/three.js/blob/bb484d616955e2bf1c3329383c3c0cc3543ccf71/examples/jsm/shaders/ParallaxShader.js#L84-L95

After merging, remember to update Differences between GLES2 and GLES3 in the documentation.

Testing project: tp.zip

Preview (Deep Parallax)

Desktop

Fedora 34 - GeForce GTX 1080 - OpenGL 2.1

HTML5

Firefox 90 - GeForce GTX 1080 - WebGL 1.0

Android (high precision not forced, resulting in artifacts)

Android 10 - OnePlus 6

Android (high precision forced)

Android 10 - OnePlus 6

Keywords for easier searching: SpatialMaterial, spatial material