Fix RGB888 to RGB565 conversion

according to https://afterthoughtsoftware.com/posts/convert-rgb888-to-rgb565
2 jobs for !209 with color-definitions in 51 seconds (queued for 3 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed build #7892

00:00:33

 
  Test
passed lint #7893

00:00:18