[HarfBuzz] harfbuzz vs uniscribe performance

[Jonathan Kew]

I've added performance numbers to the shaping comparison suite results 
at http://ec2-54-226-13-158.compute-1.amazonaws.com/index.html. The 
"timings" column lists the total number of seconds spent within 
hb_shape() for the "native" HarfBuzz OT and Uniscribe backends, and the 
percentage delta for the HB backend time compared to Uniscribe (i.e. 
negative values are good, positive are bad).

The figures should be taken with a large grain of salt, and may not 
translate directly to performance differences between HarfBuzz and 
Uniscribe when integrated into other codebases; much

Still, there are some interesting things that emerge, such as that HB 
seems to perform particularly poorly with certain fonts such as Noto 
Sans Devanagari; or that when we compare WinXP fonts with their Win8 
counterparts, HarfBuzz often runs faster with the Win8 versions, while 
Uniscribe runs substantially slower.

It might be worth investigating some of the specific cases that are 
highlighted by these results. E.g. are the relatively poor HB results 
for many WinXP fonts with "simple" scripts a result of our fallback 
positioning code, and if so can we perhaps optimize that? What is the 
particular characteristic of a number of the Noto Indic fonts that 
results in slower performance with these?

JK