add Spout semi-weekly test + vcap/spout hook

this should have catched the:
<https://github.com/CESNET/UltraGrid/issues/487>

and check also for its regression

+ added get_spout (vcap/spout) Doxy

.github/scripts/run_scheduled_tests.sh

@@ -65,6 +65,12 @@ prepare
 ##  - should_timeout - the command is expected to keep running
 ##                     (will be terminated by timeout)
 ##  - run_reflector - instead of uv, pass the args to hd-rum-transcode
+##  - Linux_only    - run just in Linux   runner
+##  - Windows_only  - "   "    "  Windows "
+##  - macOS_only    - "   "    "  macOS   "
+##
+## More platforms with "_only" suffix can be specified, however, eg.
+## Linux_only,macOS_only.
 add_test() {
         eval "test_${test_count}_args=\${1?}"
         eval "test_${test_count}_opts=\${2-}"
@@ -86,6 +92,12 @@ while [ $i -lt $test_count ]; do
                 tool=reflector
                 exec=$run_reflector
         fi
+        # skip one-platform only tests if we are not the target
+        if expr -- "$opts" : '.*_only' >/dev/null; then
+                if ! expr -- "$opts" : ".*$RUNNER_OS" >/dev/null; then
+                        continue
+                fi
+        fi
 
         timeout=5
         timeout $timeout $exec $args

.github/scripts/run_scheduled_tests_data.sh

@@ -8,6 +8,7 @@ add_test -v                                                  # basic sanity test
 add_test --nonexistent-param                   should_fail
 add_test "-d sdl"                              should_timeout
 add_test "-t testcard -c lavc:e=libx265 -f rs -d dummy" should_timeout
+add_test "-t spout:check_lib"                  Windows_only
 
 # reflector
 add_test -v                                    run_reflector # basic sanity test