GitLab

Summary:
Threaded the for loop:
```
for (int yi = 0; yi < H; ++yi) {...}
```
in function `RasterizeMeshesNaiveCpu()`.
Chunk size is approx equal.

Reviewed By: bottler

Differential Revision: D40063604

fbshipit-source-id: 09150269405538119b0f1b029892179501421e68