Internalize the EC_PROB_SHIFT precision reduction

All of the msac decode functions use 15-bit CDFs with the exception of
msac_decode_bool_prob() which takes a (15 - EC_PROB_SHIFT)-bit
probability.
This patch internalizes the reduction in precision from the EC_SMALL_MUL
experiment (hiding the define) and gives msac calls a consistent API.
16 jobs for master in 5 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style
passed #248553
amd64 debian
style-check

00:00:22

 
  Build
passed #248554
amd64 debian
build-debian

00:00:34

passed #248561
debian aarch64
build-debian-aarch64

00:01:27

passed #248562
debian aarch64
build-debian-aarch64-clang-5

00:01:06

passed #248555
amd64 debian
build-debian-static

00:00:35

passed #248564
debian aarch64
build-debian-werror

00:00:36

passed #248556
amd64 debian
build-debian32

00:00:26

passed #248563
macos
build-macos

00:00:27

passed #248559
win32
build-win-arm32

00:00:27

passed #248560
win64
build-win-arm64

00:00:27

passed #248557
win32
build-win32

00:00:31

passed #248558
win64
build-win64

00:00:34

 
  Test
passed #248565
amd64 debian
test-debian

00:00:46

passed #248566
amd64 debian
test-debian-asan

00:02:03

passed #248567
amd64 debian
test-debian-msan

00:01:03

passed #248568
amd64 debian
test-debian-ubsan

00:01:23