games-emulation/openmsx: fix build w/ newer GCC

GCC backported a change which exposed broken templates in openmsx. It's
fixed properly upstream already but add a minimal fix from Debian for now.

Closes: https://bugs.gentoo.org/958161
Signed-off-by: Sam James <sam@gentoo.org>

games-emulation/openmsx/files/openmsx-20.0-template.patch

@@ -0,0 +1,15 @@
+https://bugs.gentoo.org/958161
+https://sources.debian.org/patches/openmsx/20.0%2Bdfsg-1.2/c%2B%2B20-iterator.diff/
+
+This is fixed properly upstream in >20.0 but this is a quick, minimal fix for now.
+--- a/src/utils/view.hh
++++ b/src/utils/view.hh
+@@ -206,6 +206,7 @@ public:
+ 	}
+ 
+ 	[[nodiscard]] constexpr friend difference_type operator-(const TransformIterator& x, const TransformIterator& y)
++	requires std::sized_sentinel_for<Iterator, Iterator>
+ 	{
+ 		return x.it - y.it;
+ 	}
+

games-emulation/openmsx/openmsx-20.0-r2.ebuild

@@ -33,6 +33,11 @@ BDEPEND="${PYTHON_DEPS}"
 
 DOC_CONTENTS="If you want to if you want to emulate real MSX systems and not only the free C-BIOS machines, put the system ROMs in one of the following directories: ${EPREFIX}/usr/share/${PN}/systemroms or ~/.openMSX/share/systemroms"
 
+PATCHES=(
+	# Drop this after 20.0
+	"${FILESDIR}"/${P}-template.patch
+)
+
 src_prepare() {
 	default
 	sed -i -e 's/-fsanitize=address//' build/flavour-unittest.mk || die