If you call ./cairo-perf-print --histogram results.txt, it will then
print a histogram of the results, one per test. Ideally, you should see
a skewed distribution (with a negative skew representing that most results
run in optimal time), but random sampling errors (scheduling,
throttling, general inefficiency etc) will push it more towards a normal
distribution.

For example,
|                                                             x                |
|                                                             x xx             |
|                                                             x xx             |
|                                                             x xx             |
|                                                             xxxx             |
|                                                             xxxx x           |
|                                                          x  xxxxxx           |
|                                                          x  xxxxxx           |
|                                                          xxxxxxxxx           |
|                                                          xxxxxxxxx           |
|                                                          xxxxxxxxx           |
|                                                         xxxxxxxxxxxx         |
|                                                         xxxxxxxxxxxx         |
|                                                         xxxxxxxxxxxx         |
|                                                        xxxxxxxxxxxxxx        |
|x                                                       xxxxxxxxxxxxxx        |
|x x                                                     xxxxxxxxxxxxxxx       |
|x x                                                     xxxxxxxxxxxxxxx       |
|x x                                                    xxxxxxxxxxxxxxxxx      |
|xxx                                                 x xxxxxxxxxxxxxxxxxxx     |
|xxx                                                xxxxxxxxxxxxxxxxxxxxxxxxx  |
|xxxxxx xxxx x x x   x xxx xx xxxxx xxx x xxx x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx|
.------------------------------------------------------------------------------.
 xlib           firefox-fishtank  8298.44 1.53% (829/946)

Starts off reasonably, but quickly deteriorates as the integrated CPU/GPU
overheats and is forced to throttle.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>