ffmpeg_bridge: new shared library isolating all FFmpeg access behind a pure C API

The library lives outside of app/ and is the only component that includes
FFmpeg headers or links FFmpeg libraries. All objects (frames, packets,
decoder/encoder instances, scalers, resamplers, audio filter graphs) are
identified by opaque handles and never leave the library.

Public surface (include/ffmpeg_bridge/ffmpeg_bridge.h):
- FBFrame/FBPacket: AVFrame/AVPacket wrappers with field accessors
- FBDecoder: demux+decode instance with hwaccel fallback logic
- FBProbe: file probing, per-stream details, subtitle reading
- FBScaler/FBResampler/FBAudioGraph: swscale/swresample/avfilter wrappers
- FBEncoder: full export encoder (video filter graph, audio resampling,
  subtitles) ported from FFmpegEncoder
- FB_* constants mirror AV_* values, verified by static_asserts against
  the real FFmpeg headers inside the library

Two small bugs in the ported encoder were fixed: the resampler is now
properly freed with swr_free() (was re-initialized with swr_init() and
leaked), and codec option dictionaries are freed after avcodec_open2().

ffmpeg_bridge/CMakeLists.txt

@@ -0,0 +1,102 @@
+# Olive - Non-Linear Video Editor
+# Copyright (C) 2022 Olive Team
+# Modifications Copyright (C) 2025 mikesolar
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# ffmpeg_bridge: a self-contained shared library that isolates all FFmpeg
+# access behind a pure C API (see include/ffmpeg_bridge/ffmpeg_bridge.h).
+# The editor and the render worker link only against this library.
+
+find_package(FFMPEG 6.0 REQUIRED
+  COMPONENTS
+  avutil
+  avcodec
+  avformat
+  avfilter
+  swscale
+  swresample
+)
+
+add_library(ffmpeg_bridge SHARED
+  src/audiograph.cpp
+  src/decoder.cpp
+  src/encoder.cpp
+  src/frame.cpp
+  src/packet.cpp
+  src/probe.cpp
+  src/sws.cpp
+  src/swr.cpp
+  src/utils.cpp
+  src/internal.h
+  include/ffmpeg_bridge/ffmpeg_bridge.h
+)
+
+target_compile_features(ffmpeg_bridge PRIVATE cxx_std_17)
+
+target_include_directories(ffmpeg_bridge
+  PUBLIC
+    ${CMAKE_CURRENT_SOURCE_DIR}/include
+  PRIVATE
+    ${FFMPEG_INCLUDE_DIRS}
+)
+
+target_link_libraries(ffmpeg_bridge
+  PRIVATE
+    FFMPEG::avutil
+    FFMPEG::avcodec
+    FFMPEG::avformat
+    FFMPEG::avfilter
+    FFMPEG::swscale
+    FFMPEG::swresample
+)
+
+# Static FFmpeg (e.g. the Linux CI build) needs system libs for
+# libavcodec/libavformat.
+find_package(ZLIB REQUIRED)
+target_link_libraries(ffmpeg_bridge PRIVATE ZLIB::ZLIB)
+
+find_package(BZip2 REQUIRED)
+target_link_libraries(ffmpeg_bridge PRIVATE BZip2::BZip2)
+
+find_package(LibLZMA)
+if (LIBLZMA_FOUND)
+  target_link_libraries(ffmpeg_bridge PRIVATE ${LIBLZMA_LIBRARIES})
+endif()
+
+target_compile_definitions(ffmpeg_bridge PRIVATE FFMPEG_BRIDGE_BUILD)
+
+# Only export the FB_API functions
+set_target_properties(ffmpeg_bridge PROPERTIES
+  C_VISIBILITY_PRESET hidden
+  CXX_VISIBILITY_PRESET hidden
+  VISIBILITY_INLINES_HIDDEN ON
+)
+
+# The shared library lives in its own directory outside of app/
+set_target_properties(ffmpeg_bridge PROPERTIES
+  LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/ffmpeg_bridge/bin"
+  RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/ffmpeg_bridge/bin"
+  ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/ffmpeg_bridge/lib"
+)
+
+if (WIN32)
+  set_target_properties(ffmpeg_bridge PROPERTIES PREFIX "")
+endif ()
+
+install(TARGETS ffmpeg_bridge
+  RUNTIME DESTINATION ffmpeg_bridge/bin
+  LIBRARY DESTINATION ffmpeg_bridge/bin
+  ARCHIVE DESTINATION ffmpeg_bridge/lib
+)