packaging: ship the Vulkan backend on all platforms, standard lib layout

- Linux CD builds no longer disable Vulkan: the AppImage, deb, rpm and
  Arch packages now build and ship liboakvulkan (the AppImage deploys
  it via linuxdeploy --library so libvulkan is bundled too, and the
  verify step checks both backend libraries); deb/rpm dependencies gain
  libvulkan1/vulkan-loader; the Arch PKGBUILD gains vulkan-headers
- macOS CD installs vulkan-loader and exports VULKAN_SDK so
  find_package(Vulkan) locates the Homebrew loader (previously the
  Vulkan backend silently never built there)
- ffmpeg_bridge now installs to the standard lib directory with
  /../lib RPATH instead of the non-standard ffmpeg_bridge/bin
  layout (verified: editor, worker, oakgl and oakvulkan all resolve it)
- build guides (EN/ZH) document the macOS Vulkan backend dependencies
  and the VULKAN_SDK variable

ffmpeg_bridge/CMakeLists.txt

@@ -101,8 +101,8 @@ if (WIN32)
   )
 else()
   install(TARGETS ffmpeg_bridge
-    RUNTIME DESTINATION ffmpeg_bridge/bin
-    LIBRARY DESTINATION ffmpeg_bridge/bin
+    RUNTIME DESTINATION bin
+    LIBRARY DESTINATION lib
     ARCHIVE DESTINATION ffmpeg_bridge/lib
   )
 endif ()