Improve motion derivative zero finding.
Improve motion derivative zero finding.

When applying Newton's method to find times at which motion is at an extremum:
1. Only accept times that are inside the t interval the current call to IntervalFindZeros() is repsonsible for. This fixes bugs where many zeros would be found for motion functions that were almost flat over a section of their domian.
2. However, still accept points that are slightly outside of the interval to allow for floating-point roundoff error.

bssrdf/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,bssrdf/pbrt-v3,bssrdf/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,mwkm/pbrt-v3,ambakshi/pbrt-v3,mmp/pbrt-v3,ambakshi/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3,mmp/pbrt-v3,mwkm/pbrt-v3