disable r_depthPrepass by default for performance

[VierEck]

i observed a 30 fps increase whenever r_depthPrepass was toggled off on latest release (Revision 12). heres a vid: https://github.com/nonperforming/openspadesplus/assets/85409780/215455fd-f8e3-400d-ba78-c77d27f8eca3

it seems by disabling r_depthPrepass we can skip this map prerenderer which is probably why the performance improves:

[nonperforming]

Interesting, thanks!