README: add screenshot of plplay

It would be cool if I could figure out more screenshots to add, maybe
some sort of comparison between bilinear and jinc scaling or w/e.
12 jobs for master in 8 minutes and 12 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Compile
passed aarch64 #603196
docker aarch64

00:00:09

passed linux #603192
docker amd64

00:00:09

passed macos #603197
amd64 catalina

00:00:07

passed scan #603198
docker amd64

00:00:34

passed static #603193
docker amd64

00:00:10

passed win32 #603194
docker amd64

00:00:11

passed win64 #603195
docker amd64

00:00:09

 
  Test
passed gpu #603200
gpu

00:01:32

79.1142%
passed llvmpipe #603199
docker amd64

00:00:27

 
  Sanitize
passed asan #603201
gpu

00:01:59

passed msan #603202
gpu

00:01:54

passed ubsan #603203
gpu

00:01:43