Slightly over-allocate picture buffers

Allows simplified SIMD function implementations that don't exactly
respect picture boundaries when reading picture data. Fixes #251 and
#250.
20 jobs for master in 6 minutes and 51 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Style
passed #257147
amd64 debian
style-check

00:00:20

 
  Build
passed #257148
amd64 debian
build-debian

00:00:38

passed #257156
debian aarch64
build-debian-aarch64

00:01:28

passed #257157
debian aarch64
build-debian-aarch64-clang-5

00:01:00

passed #257160
debian armv7
build-debian-armv7

00:01:30

passed #257161
debian armv7
build-debian-armv7-clang-5

00:01:09

passed #257149
amd64 debian
build-debian-static

00:00:35

passed #257159
debian aarch64
build-debian-werror

00:00:33

passed #257150
amd64 debian
build-debian32

00:00:28

passed #257158
macos
build-macos

00:00:30

passed #257154
amd64 debian
build-win-arm32

00:00:26

passed #257155
amd64 debian
build-win-arm64

00:00:33

passed #257151
amd64 debian
build-win32

00:00:38

passed #257152
amd64 debian
build-win32-unaligned-stack

00:00:31

passed #257153
amd64 debian
build-win64

00:00:44

 
  Test
passed #257162
amd64 debian
test-debian

00:00:50

passed #257163
amd64 debian
test-debian-asan

00:02:02

passed #257164
amd64 debian
test-debian-msan

00:01:06

passed #257165
amd64 debian
test-debian-ubsan

00:01:26

passed #257166
amd64 debian
test-win64

00:00:49