proxy: isolate proxies from export renders and improve proxy workflow

- FootageJob::should_use_proxy() centralizes the proxy decision; worker
  pre-decode now honors the render mode so exports always decode the
  original media (previously every frame was pre-decoded from proxies)
- global Tools > Use Proxy Media toggle with footage invalidation
- ffmpeg -progress parsing for real percentage feedback while generating
- divider mode (1/2, 1/4, 1/8 of source resolution) with UI, proxy
  filename tags and per-footage persistence (pdivider)
- Media Offline warning slat rendered for missing footage
- regression tests: export isolation, relink invalidation, offline slat,
  progress parsing, divider arguments

tests/gtest/CMakeLists.txt

@@ -46,6 +46,7 @@ add_executable(olive-gtest
   project_serializer_test.cpp
   proxy_manager_test.cpp
   proxy_dialog_test.cpp
+  footage_job_proxy_test.cpp
   lut_file_field_test.cpp
   node_math_test.cpp
   node_undo_test.cpp
@@ -184,6 +185,12 @@ if (OAK_ENABLE_DYNAMIC_RENDER_BACKEND)
   endif()
 endif()
 
+# Several render tests spawn the render worker binary from the build tree; it
+# must be up to date with the code under test
+if (TARGET olive-render-worker)
+  add_dependencies(olive-gtest olive-render-worker)
+endif()
+
 if (MSVC)
   add_test("Olive.gtest" olive-gtest)
 else()