`feature_frame_limit` always returns a value once configured

[Athosvk]

For clarity, my reference is changing the settings through the driver control panel

Once a value for the frame limit control is configured, this function will always return whichever max fps was configured, even if the feature itself is toggled off. I suppose the more logical interface here would be to either return Err if the feature is disabled or be an Option to see if the feature itself is toggled on/off.

Unfortunately I don't immediately see a way to actually query this: the docs don't make mention of a separate value that we can query as to whether this is toggled on or not, nor does there seem to be a way to pass this alongside ctl_3d_feature_getset_t as is.