Build OpenCV with FFmpeg on Linux

[c-ewing]

Description

Due to the non-standard install location of FFmpeg during the AliceVision build OpenCV cannot find it. This patch adds FFmpeg to the BUILD_PREFIX of OpenCV to enable the FFmpeg features.

This builds on the work done in #1732 by @furbrain and @fabiencastan.

Features list

• Fix [bug] KeyframeSelection node not opening video Meshroom#2285

Implementation remarks

Expanding on #1732, OpenCV fetches a prebuilt FFmpeg when building for Windows but looks for system libraries when building on Linux.

OpenCV being built without FFmpeg is confirmed by building the Ubuntu docker image and running:

$ ./opt/AliceVision_install/bin-deps/opencv_version -v | grep -i ffmpeg

Which responds with NO.

OpenCV is picky about how external dependencies are specified and it does not support directly specifying install locations. Instead of specifying a location OpenCV's CMAKE uses find_package() to find packages. It first looks for Modules in CMAKE_MODULE_PATH, then looks within the OpenCV source for Find.cmake files according to the CMake Docs.

This implementation adds the FFmpeg install path to the ${CMAKE_PREFIX_PATH} of OpenCV, allowing find_package() to search in ${CMAKE_INSTALL_PREFIX} when building.

This change is confirmed by running opencv_version -v | grep -i ffmpeg, resulting in YES

[cbentejac]

Thanks for your contribution @c-ewing!