Add support for FFmpeg on Windows

[hwangsihu]

Fixes #776

[bear101]

@hwangsihu I cannot test this branch because it is not possible to build debug versions of anything that depends on tinyxml2:

2>------ Build started: Project: catchtt, Configuration: Debug x64 ------
2>tinyxml2d.lib(tinyxml2.obj) : error LNK2038: mismatch detected for '_ITERATOR_DEBUG_LEVEL': value '0' doesn't match value '2' in CatchDefault.obj
2>tinyxml2d.lib(tinyxml2.obj) : error LNK2038: mismatch detected for 'RuntimeLibrary': value 'MT_StaticRelease' doesn't match value 'MTd_StaticDebug' in CatchDefault.obj

It looks like TinyXML2 uses the release runtime library for building it's debug library. In other words the CMAKE_MSVC_RUNTIME_LIBRARY does not create a debug and release version of TinyXML2.