kwin: allow kwin to set real-time thread priotiry

KWin supports setting RT thread priorities to improve rendering and
input processing speed when the system is under load for example. It
should also result in a smoother experience in general.

As an added bonus, we also get rid of this warning:

kwin_wayland_wrapper[1016]: Failed to gain real time thread priority (See CAP_SYS_NICE in the capabilities(7) man page). error: Operation not permitted

See: https://blog.martin-graesslin.com/blog/2017/09/kwinwayland-goes-real-time/

Signed-off-by: Marc Ferland <marc.ferland@sonatest.com>

recipes-plasma/kwin.inc

@@ -36,6 +36,7 @@ DEPENDS:class-target = " \
     kxmlgui \
     lcms \
     libdisplay-info \
+    libcap \
     libei \
     libinput \
     libplasma \
@@ -88,6 +89,11 @@ do_install:append() {
     install -m 644 ${UNPACKDIR}/kwinrc ${D}/${sysconfdir}/xdg/kwinrc
 }
 
+pkg_postinst:${PN} () {
+	setcap cap_sys_nice+ep "$D/${bindir}/kwin_wayland"
+}
+PACKAGE_WRITE_DEPS += "libcap-native"
+
 FILES_SOLIBSDEV = ""
 FILES:${PN} += " \
     ${datadir}/config.kcfg/*.kcfg \