Use vendored plutovg, and do not install plutovg.

Index: rts/lib/lunasvg/CMakeLists.txt
--- rts/lib/lunasvg/CMakeLists.txt.orig
+++ rts/lib/lunasvg/CMakeLists.txt
@@ -6,10 +6,7 @@ set(LUNASVG_VERSION_MICRO 1)
 
 project(lunasvg LANGUAGES CXX VERSION ${LUNASVG_VERSION_MAJOR}.${LUNASVG_VERSION_MINOR}.${LUNASVG_VERSION_MICRO})
 
-find_package(plutovg 0.0.4 QUIET)
-if(NOT plutovg_FOUND)
-    add_subdirectory(plutovg)
-endif()
+add_subdirectory(plutovg EXCLUDE_FROM_ALL)
 
 set(lunasvg_sources
     source/lunasvg.cpp
