Fix length() array function usage in shader

[Chaosus]

I think I broke it in #48933:

Update:
Also fixes incorrect array element size returning from the arrays:

[lawnjelly]

Just out of interest because I don't know when this was put in, how come it is length and not size, as size seems to be used throughout Godot? (that's presuming length is to give how many elements in the array)

[Chaosus]

@lawnjelly it's from GLSL, I don't know whether it's a good idea to modify it to size - it may break user's shaders

[lawnjelly]

@lawnjelly it's from GLSL, I don't know whether it's a good idea to modify it to size - it may break user's shaders

Ah if it is GLSL standard, then agree that is best to stick with, even if different from Godot. 👍

[lawnjelly]

Looks fine to me (although I'm not up on the shader compiler in 4.x).