merge 0,1,2,3,... and 0.5f

Because floats are fun, the compiler cannot merge x + 0.5f +
[0,1,2,3,4...] into x + [0.5,1.5,2.5,3.5,4.5,...].  But we can.

Change-Id: I03b46c1ea0653877f35f6c888f29371b5f73d813
Reviewed-on: https://skia-review.googlesource.com/42480
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed