demos/colors: time colors to system clock

Rather than the rendering framerate. Otherwise it behaves quite funnily
when e.g. resizing the window or stuff like that.
12 jobs for master in 8 minutes and 15 seconds (queued for 1 second)
Status Job ID Name Coverage
  Compile
passed #639344
aarch64 docker
aarch64

00:00:09

passed #639340
amd64 docker
linux

00:00:11

passed #639345
catalina amd64
macos

00:00:07

passed #639346
amd64 docker
scan

00:00:27

passed #639341
amd64 docker
static

00:00:09

passed #639342
amd64 docker
win32

00:00:17

passed #639343
amd64 docker
win64

00:00:21

 
  Test
passed #639348
gpu
gpu

00:02:00

78.4315%
passed #639347
amd64 docker
llvmpipe

00:00:34

 
  Sanitize
passed #639349
gpu
asan

00:01:59

passed #639350
gpu
msan

00:01:43

passed #639351
gpu
ubsan

00:02:03