Add tile constraint to loopfilter mask fix-up code

Fixes #92.
8 jobs for master in 2 minutes and 30 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #5181
debian amd64

00:00:30

passed build-debian-aarch64 #5185
aarch64 debian

00:01:39

passed build-debian-aarch64-clang-5 #5186
aarch64 clang5 debian

00:01:15

passed build-debian-static #5182
debian amd64

00:00:29

passed build-debian-werror #5188
debian

00:01:12

passed build-macos #5187
macos

00:00:24

passed build-win32 #5183
win32

00:00:26

passed build-win64 #5184
win64

00:00:33