Don't call mask_edges_chroma when !bw4 or !bh4

Fix issue #70
8 jobs for master in 2 minutes and 10 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #5561
debian amd64

00:00:30

passed build-debian-aarch64 #5565
aarch64 debian

00:01:35

passed build-debian-aarch64-clang-5 #5566
aarch64 clang5 debian

00:01:19

passed build-debian-static #5562
debian amd64

00:00:31

passed build-debian-werror #5568
aarch64 debian

00:00:51

passed build-macos #5567
macos

00:00:24

passed build-win32 #5563
win32

00:00:28

passed build-win64 #5564
win64

00:00:34