fix: fix inconsistent VR grass culling

[thorwin99]

Potential fix for #124.
VR grass cull distance was calculated per eye, resulting in different cull distances for each eye.
This meant, that grass might be visible in the left eye, while culled in the right eye.
This is a potential fix, calculating the cull distance based on the first eye.

[alandtse]

Thanks. Do you have any confirmation this actually works like you expect? Please note, this will impact flat.

[thorwin99]

Yes, i just tested it with my VR setup. It simply takes the WorldViewProj matrix of the first eye (eyeIndex 0) for culling distance calculation of every eye. That way, the culling distance is consistent for both eyes,

It wont impact flat, as flat already uses eyeIndex 0 by default, which results in the same calculation.
I thought about adding a switch for VR / Non VR, but that would be unnecessary.

[alandtse]

@doodlum you ok with this? I'm good with it.

[doodlum]

👀 Yes this is fine. It should only apply to VR, though.