Sanitize CUS_GL4 clip planes shader code

[lhog]

Description

There're a few issues with CUS shader:

This code uses gl_ClipDistance array, but it's not declared in the vertex shader like (as it should):
out float gl_ClipDistance[3];

https://github.com/beyond-all-reason/Beyond-All-Reason/blob/master/modelmaterials_gl4/templates/defaultMaterialTemplate.lua#L512 and https://github.com/beyond-all-reason/Beyond-All-Reason/blob/master/modelmaterials_gl4/templates/defaultMaterialTemplate.lua#L516 deal with gl_ClipDistance[2] and gl_ClipDistance[0] respectively, gl_ClipDistance[1] is dangling.

https://github.com/beyond-all-reason/Beyond-All-Reason/blob/master/modelmaterials_gl4/templates/defaultMaterialTemplate.lua#L115-L116 are unused, which is confusing.

I believe these issues combined make models look like this on Zink (OpenGL over Vulkan driver):

Expected Behaviour

See above

Actual Behaviour

See above

Reproduction steps

No response

Other

No response