ImageColors: use OpenMP to position colors in parallel
The color positioning loop is the most heavy part in generatePalette, so make it possible to position colors in parallel when the valid sample size is larger than 256x256 (65536) before one CPU core gets exhausted.
Test:
| Source Image | Total Generation Time Before (ms) | After (ms) |
|---|---|---|
 |
7616 | 2121 |
Edited by Fushan Wen