ci: fix three platform-specific build failures

- Linux: Qt6 metatype static_assert requires complete types; declare
  OakEngineBlock with Q_DECLARE_OPAQUE_POINTER in the public header
  (outside extern "C")
- Windows: gtest discovery runs the freshly linked test exe at build
  time and it cannot find the DLLs (0xc0000135); copy oakcore/oakengine/
  ffmpeg_bridge DLLs next to the test binaries on WIN32
- macOS: Imath header mixing between otio-install's stale bundled Imath
  and Homebrew's Imath broke ImathBox.h (V2h undeclared); bump the OTIO
  cache to v2 and drop bundled Imath headers so the system one is the
  only set in play

core/tests/CMakeLists.txt

@@ -32,3 +32,10 @@ target_include_directories(oakcore_gtest PRIVATE
 )
 
 gtest_discover_tests(oakcore_gtest)
+
+if (WIN32)
+  # gtest discovery executes the binary at build time; it needs the DLL next to it
+  add_custom_command(TARGET oakcore_gtest POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different
+            $<TARGET_FILE:olivecore> $<TARGET_FILE_DIR:oakcore_gtest>)
+endif ()