Allow `smsize 15` for better shadow map resolution

[Calinou]

This results in sharper, more stable shadows that better connect to the object casting them (with less acne/peter-panning).

This is a demanding setting compared to /smsize 14, but modern high-end GPUs[^1] can still render the game smoothly with /smsize 15 in most maps. Still, given how demanding this setting is, it's intentionally not exposed in the graphics options menu.

I also tried exposing /smsize 16 for fun, but it looks the same as /smsize 15 – presumably because GPUs don't support such large textures in the first place. (I'm surprised /smsize 15 even works here…; I remember when smoothly using /smsize 14 was just a pipe dream.)

[^1]: Roughly RTX 3080 level in terms of rasterization performance or better.