crf-search: 2nd iteration 20/80 & disable on small min/max crf ranges

[alexheretic]

• crf-search: Tweak 2nd iteration logic that slices the crf range at the 25% or 75% crf point.
  • Widen to 20%/80% to account for searches of the "middle" two subranges being more optimal.
  • Disable when using custom min/max crf ranges under half the default.

The 2nd iter logic is a bit arcane and not at all obvious to the user. In future this should probably be configurable.

Relates to discussions in #227, #232

Example

So for default settings:

1. Try 32
2. Assuming 32 failed, try 19 (previously 21)

• If failed try min-crf = 10
• If success linear interpolate next crf between in range (19, 32)

The slight widening means more searches will get to linear interpolation on 3rd iter (those that 21 would have failed on). Though those 21 would have passed on will have slightly worse linear interpolated guess for 3rd iter.

The ones that still fail at 19 will continue to next try min-crf=10, but will benefit slightly on 4th iter with slightly tighter range to interpolate inside.