Fix OCIO LUT async processor black screen and freeze

- Pass through input texture when the LUT processor is not ready yet,
  preventing black frames while the processor is being generated.
- Serialize processor generation with a single in-flight task to avoid
  concurrent OCIO lock contention that could freeze the UI.
- Invalidate cached frames after the async processor is set so the viewer
  refreshes automatically without requiring the playhead to be moved.
- Add OAK_DISABLE_HWACCEL environment variable to force software decoding.
- Add FFmpegDecoderHW regression test for H.264 4:2:2 10-bit decoding.

tests/gtest/CMakeLists.txt

@@ -38,6 +38,7 @@ add_executable(olive-gtest
   plugin_ofx_integration_test.cpp
   codec_frame_test.cpp
   codec_decoder_test.cpp
+  ffmpeg_decoder_hw_test.cpp
   codec_exportcodec_test.cpp
   codec_exportformat_test.cpp
   codec_encoder_test.cpp