Require refs and curpic to share the same chroma subsampling

Fixes #90.
8 jobs for master in 2 minutes and 25 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #5053
debian amd64

00:00:31

passed build-debian-aarch64 #5057
aarch64 debian

00:01:41

passed build-debian-aarch64-clang-5 #5058
aarch64 clang5 debian

00:01:19

passed build-debian-static #5054
debian amd64

00:00:29

passed build-debian-werror #5060
debian

00:01:05

passed build-macos #5059
macos

00:00:24

passed build-win32 #5055
win32

00:00:27

passed build-win64 #5056
win64

00:00:32