tiny_bvh_fenster: Output frame times to stderr.

[eloj]

$ ./tiny_bvh_fenster
Frame 1 time: 1363 ms, avg: 1363.00 ms
Frame 2 time: 1356 ms, avg: 1359.50 ms
Frame 3 time: 1361 ms, avg: 1360.00 ms
Frame 4 time: 1324 ms, avg: 1351.00 ms
Frame 5 time: 1324 ms, avg: 1345.60 ms
Frame 6 time: 1319 ms, avg: 1341.17 ms
Frame 7 time: 1334 ms, avg: 1340.14 ms
...

Alternatively it could be moved into the run-loop of fenster.h, but there's only one use now so...

[eloj]

Again I didn't catch that you were already working on this.

[jbikker]

I want to add a fourth source just for profiling, which will be text-only but it will use the same set of rays as the fenster renderer. Tracing the rays is already from a buffer; this way I can easily do accurate timing:

• Run it once or twice to warm the caches
• Run it three times and average to get a good estimate of performance
• Run it three times with OpenMP to get threaded performance.
  Once this is in place I can start adding more advanced BVH algorithms; the profiling tool will then serve to show the gains (and build performance cost) of each approach.