ci: fix macOS/Linux configure (oakgl) and Windows MSYS2 git

- OAK_ENABLE_DYNAMIC_RENDER_BACKEND now defaults ON; it was previously
  undefined (OFF) unless passed, so CI configure failed on the
  unconditional oakgl target references in app/worker/tests
- add git to the MSYS2 package list in ci.yml and cd.yml; the OTIO
  build step runs git clone in the msys2 shell, where git is not
  installed by default

engine/CMakeLists.txt

@@ -147,6 +147,8 @@ endif ()
 # Dynamic render backends (OpenGL/Vulkan) are engine plugins loaded via
 # dlopen; they share this engine library instead of embedding a static
 # render-core subset.
+option(OAK_ENABLE_DYNAMIC_RENDER_BACKEND
+       "Build OpenGL/Vulkan render backends as dlopen plugins (oakgl/oakvulkan)" ON)
 if (OAK_ENABLE_DYNAMIC_RENDER_BACKEND)
     target_compile_definitions(oakengine-obj PRIVATE OAK_ENABLE_DYNAMIC_RENDER_BACKEND)