In order to handle 'cairo-perf-trace benchmark', we need to perform the
can_run? test on the directory name as opposed to the individual trace
names. Make it so.
By ensuring that tests take longer than a couple of seconds we eliminate
systematic errors in our measurements. However, we also effectively
eliminate the synchronisation overhead. To compensate, we attempt to
estimate the overhead by reporting the difference between a single
instance and the minimum averaged instance.
Use the new API Behdad exposed in 1.8 to precompute a glyph string using
Cairo and then benchmark cairo_show_glyphs(). This is then equivalent to
the text benchmark but without the extra step of converting to glyphs on
every call to cairo_show_text() i.e. it shows the underlying glyph
rendering performance.
