Using a fixed point FontUnit dimension for internal calculations.

[cbreeden]

On IRC we discussed the possibility of using a scale of internal font units (in xits it's 1/1000 em) for internal dimension in ReX. We concluded that it would probably be best if we use a 24.8 fixedpoint representation of the internal font unit. A cursory glance of the code showed that *2, /2, and +/- is by far the most common operation. However, *0.68 is used for scripts scaling, and it is unclear what the distribution of *0.68 use is to make a fully informed decision.

Pertinent numbers from the intel optimization manual:

	Latency	Throughput
ADD	1	0.5
MUL	7,8	6,7
SHL	1	1
FADD	5	4,1
FMUL	5	1

Considering the number of *2, /2 uses, it seems unlikely that f64 will afford the same bitshift optimizations that a u32 would. FDIV is not listed, but expensive.

sebk also pointed out that it could be possible to put a scaling call in both the layout and renderer, but depending on a feature flag, one would be a no op and the other wouldn't.

Using a scaled font unit would also rid of all the Pixels -> FontUnit conversions in the Layout (there seem to be quite a few).