Allow to specify custom quality settings when building target

[NickAcPT]

Hey there, first of all, I want to thank you for this port, it has been very useful so far!
I've been running it in production for my Minecraft skin avatar service for a while now and it's been smooth sailing.

Right now, I've running it with the Ultra quality setting, but since I wanted to lower the threshold (from 0.05 down to 0.01 - it might be placebo lol), I've had to keep a fork laying around.

As such, i was wondering if it'd be possible to introduce a "Custom" quality setting whose values (SMAA_THRESHOLD, SMAA_MAX_SEARCH_STEPS, SMAA_MAX_SEARCH_STEPS_DIAG, SMAA_CORNER_ROUNDING) could be provided from within Rust.

Thank you for your time.