Add thumbnail support via ffmpeg API (incomplete

extensions/cmake/FindAVCODEC.cmake

@@ -0,0 +1,33 @@
+find_package(PkgConfig)
+if (PkgConfig_FOUND)
+    pkg_check_modules(PC_AVCODEC QUIET IMPORTED_TARGET GLOBAL libavcodec)
+endif()
+
+if (PC_AVCODEC_FOUND)
+    set(AVCODEC_FOUND TRUE)
+    set(AVCODEC_VERSION ${PC_AVCODEC_VERSION})
+    set(AVCODEC_VERSION_STRING ${PC_AVCODEC_STRING})
+    set(AVCODEC_LIBRARYS ${PC_AVCODEC_LIBRARIES})
+    if (USE_STATIC_LIBS)
+        set(AVCODEC_INCLUDE_DIRS ${PC_AVCODEC_STATIC_INCLUDE_DIRS})
+    else()
+        set(AVCODEC_INCLUDE_DIRS ${PC_AVCODEC_INCLUDE_DIRS})
+    endif()
+    if (NOT AVCODEC_INCLUDE_DIRS)
+        find_path(AVCODEC_INCLUDE_DIRS NAMES libavcodec/avcodec.h)
+    endif()
+    if (NOT TARGET AVCODEC::AVCODEC)
+        add_library(AVCODEC::AVCODEC ALIAS PkgConfig::PC_AVCODEC)
+    endif()
+else()
+    message(FATAL_ERROR "failed.")
+endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(AVCODEC
+    FOUND_VAR AVCODEC_FOUND
+    REQUIRED_VARS
+        AVCODEC_LIBRARYS
+        AVCODEC_INCLUDE_DIRS
+    VERSION_VAR AVCODEC_VERSION
+)