ci: declare all oakengine handle types opaque for Qt metatypes; defer gtest discovery on Windows

- Linux GCC rejects QMetaType instantiation over incomplete types
  (static_assert(sizeof(T))); Q_DECLARE_OPAQUE_POINTER every opaque
  OakEngine* handle in the public headers so QList<OakEngineTask*> etc
  compile
- Windows: gtest discovery ran the fresh exe before the DLLs were next
  to it; use DISCOVERY_MODE PRE_TEST on WIN32 and copy the DLLs for
  ctest time

engine/CMakeLists.txt

@@ -291,10 +291,11 @@ if (BUILD_TESTS)
         endif ()
         # Freshly linked binaries can exceed the 5s default discovery
         # timeout on first run (dyld cold cache + Qt/OCIO init)
-        gtest_discover_tests(${name} DISCOVERY_TIMEOUT 60)
         if (WIN32)
-            # gtest discovery executes the binary at build time; it needs
-            # the engine DLLs next to it
+            # Defer discovery to ctest time; at build time the DLLs are not
+            # next to the exe yet (copied below).
+            gtest_discover_tests(${name} DISCOVERY_TIMEOUT 60
+                                 DISCOVERY_MODE PRE_TEST)
             add_custom_command(TARGET ${name} POST_BUILD
                 COMMAND ${CMAKE_COMMAND} -E copy_if_different
                         $<TARGET_FILE:oakengine> $<TARGET_FILE_DIR:${name}>
@@ -302,6 +303,8 @@ if (BUILD_TESTS)
                         $<TARGET_FILE:olivecore> $<TARGET_FILE_DIR:${name}>
                 COMMAND ${CMAKE_COMMAND} -E copy_if_different
                         $<TARGET_FILE:ffmpeg_bridge> $<TARGET_FILE_DIR:${name}>)
+        else ()
+            gtest_discover_tests(${name} DISCOVERY_TIMEOUT 60)
         endif ()
     endfunction()