[Pixman] Questionable numbers from lowlevel-blt-bench

[Matt Turner]

Hi Jonathan,

In doing performance work, I've noticed some weird results from
lowlevel-blt-bench. Often it has seemed that the RT results determined
the Kops/s almost entirely. I came across an instance of this today
which was particularly striking:

Before:
add_8888_8888 =  L1:  47.01  L2:  36.84  M: 18.96 ( 33.14%)  HT: 35.94
 VT: 33.82  R: 30.64  RT: 19.36 ( 181Kops/s)

After:
add_8888_8888 =  L1: 230.78  L2: 200.86  M: 90.48 (159.44%)  HT: 48.41
 VT: 45.46  R: 42.78  RT: 19.22 ( 181Kops/s)

L1/L2/M numbers are improved by ~5x. HT, VT, and R numbers are
improved by ~1.35x. RT doesn't change... neither does Kops/s.

What's going on here, and can we make the composite result more sensible?

Thanks,
Matt