ci: engine tests use dllexport-consistent macros on Windows; run Linux tests under Xvfb

- make_oakengine_gtest embeds oakengine-obj; test code must see
  OAKENGINE_API as dllexport too (OAKENGINE_BUILD), otherwise MinGW
  resolves nothing against __imp_* stubs
- the render worker intentionally drops QT_QPA_PLATFORM=offscreen and
  needs a real display for its GL context; run the Linux ctest and the
  filtered gtest steps under xvfb-run

.github/workflows/ci.yml

@@ -40,7 +40,8 @@ jobs:
             qt6-base-dev qt6-base-dev-tools qt6-base-private-dev qt6-tools-dev qt6-tools-dev-tools \
             libavcodec-dev libavformat-dev libavutil-dev libswscale-dev libswresample-dev libavfilter-dev \
             libopencolorio-dev libopenimageio-dev libopenexr-dev libexpat1-dev \
-            portaudio19-dev libgl1-mesa-dev libxkbcommon-dev ccache
+            portaudio19-dev libgl1-mesa-dev libxkbcommon-dev ccache \
+            xvfb
 
       # ------------------------------------------------------------------
       # macOS dependencies
@@ -248,7 +249,7 @@ jobs:
         if: runner.os == 'Linux'
         env:
           QT_QPA_PLATFORM: offscreen
-        run: ctest --test-dir build --output-on-failure -C ${{ env.CMAKE_BUILD_TYPE }}
+        run: xvfb-run -a ctest --test-dir build --output-on-failure -C ${{ env.CMAKE_BUILD_TYPE }}
 
       - name: Test (macOS)
         if: runner.os == 'macOS'
@@ -266,7 +267,7 @@ jobs:
         if: runner.os == 'Linux'
         env:
           QT_QPA_PLATFORM: offscreen
-        run: ./build/tests/gtest/olive-gtest --gtest_filter="PluginSmoke*"
+        run: xvfb-run -a ./build/tests/gtest/olive-gtest --gtest_filter="PluginSmoke*"
 
       - name: Plugin Smoke Tests (macOS)
         if: runner.os == 'macOS'
@@ -281,7 +282,7 @@ jobs:
         if: runner.os == 'Linux'
         env:
           QT_QPA_PLATFORM: offscreen
-        run: ./build/tests/gtest/olive-gtest --gtest_filter="PluginIntegration.*:PluginMisc.*"
+        run: xvfb-run -a ./build/tests/gtest/olive-gtest --gtest_filter="PluginIntegration.*:PluginMisc.*"
 
       - name: OFX Integration Tests (macOS)
         if: runner.os == 'macOS'
@@ -291,7 +292,7 @@ jobs:
         if: runner.os == 'Linux'
         env:
           QT_QPA_PLATFORM: offscreen
-        run: ./build/tests/gtest/olive-gtest --gtest_filter="AudioSmoke*"
+        run: xvfb-run -a ./build/tests/gtest/olive-gtest --gtest_filter="AudioSmoke*"
 
       - name: Audio Smoke Tests (macOS)
         if: runner.os == 'macOS'
@@ -306,7 +307,7 @@ jobs:
         if: runner.os == 'Linux'
         env:
           QT_QPA_PLATFORM: offscreen
-        run: ./build/tests/gtest/olive-gtest --gtest_filter="ViewerSmoke*"
+        run: xvfb-run -a ./build/tests/gtest/olive-gtest --gtest_filter="ViewerSmoke*"
 
       - name: Viewer Smoke Tests (macOS)
         if: runner.os == 'macOS'

engine/CMakeLists.txt

@@ -309,6 +309,12 @@ if (BUILD_TESTS)
         if (UNIX AND NOT APPLE)
             target_link_options(${name} PRIVATE "LINKER:--export-dynamic")
         endif ()
+        if (WIN32)
+            # The test embeds oakengine-obj; its OAKENGINE_API references must
+            # match the library side (dllexport), otherwise MinGW looks for
+            # __imp_* import stubs that only a real DLL import lib provides.
+            target_compile_definitions(${name} PRIVATE OAKENGINE_BUILD)
+        endif ()
         # Freshly linked binaries can exceed the 5s default discovery
         # timeout on first run (dyld cold cache + Qt/OCIO init)
         if (WIN32)