Fix conversion of video frames to RGB

Video frames aren't actually BGRA; the alpha channel is actually just
garbage since CSRI uses 0 for opaque and other things use 255. To work
around this, add a custom colorspace converter.

Fixes the subtitles preview in the style editor and copying frames to
the clipboard/saving them.

Closes #1621.
Use -Wno-c++11-narrowing when available

Needed to build boost.gil with Apple Clang 5.0