The build fails as:
```
src/zimg/colorspace/matrix3.cpp:20:7:
error: 'size_t' was not declared in this scope; did you mean 'std::size_t'?
20 | for (size_t i = 0; i < 3; ++i) {
| ^~~~~~
| std::size_t
```
Direct backport of upstream
9ae36d7d5f74 ("Update matrix3.cpp")
which adds missing <cstddef> include needed for 'size_t'.
Reported-by: Patrick McLean
Closes: https://bugs.gentoo.org/721676
Package-Manager: Portage-2.3.99, Repoman-2.3.22
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
--- /dev/null
+https://bugs.gentoo.org/721676
+
+From 9ae36d7d5f7420eaacd9644451933512fa13d716 Mon Sep 17 00:00:00 2001
+From: sekrit-twc <sekrit-twc@users.noreply.github.com>
+Date: Wed, 4 Dec 2019 12:01:26 -0800
+Subject: [PATCH] Update matrix3.cpp
+
+---
+ src/zimg/colorspace/matrix3.cpp | 1 +
+ 1 file changed, 1 insertion(+)
+
+--- a/src/zimg/colorspace/matrix3.cpp
++++ b/src/zimg/colorspace/matrix3.cpp
+@@ -1,3 +1,4 @@
++#include <cstddef>
+ #include "matrix3.h"
+
+ namespace zimg {
SLOT="0"
IUSE="cpu_flags_x86_sse debug static-libs"
+PATCHES=("${FILESDIR}"/${PN}-2.9.2-gcc-10.patch)
+
src_prepare() {
default
eautoreconf