Fix occlusion culling buffer getting overwritten in larger scenes

[Rudolph-B]

Fixes #100032

On subsequent calls to _transform_vertices_range() (where p_from would have been adjusted) it would still start writing to the same index in of xformed_vertices effectively overwriting and corrupting vertex data previously written.

The correct implementation is to offset p_write with 3*p_from.

I tested using occlusion_culling_mesh_lod for both double and float builds.

Before:

After:

[clayjohn]

Makes sense!

[Repiteo]

Thanks!