Reduce ProgressReporter update frequency after 15m.
Reduce ProgressReporter update frequency after 15m.

This reduces the size of log files that include progress output for long
renders.

nispaur/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3