More shader detail level options in graphics options

[h106frp]

We currently have shaders ON/OFF as a graphics option. Would it be possible to change this to a slider that shifts the base MIPMAP used.

i.e. max = full shader map, step 1 reduces this to MIPMAP level 1 (reduced by ^2) or 1/4 size etc.

This would allow tracks to be be built with a bit of future proofing of higher shader maps sizes but allow lower spec graphics hardware to improve framerate but still have some shading.

Thank you