Proper Audio/Video Dumping

Screwed up the previous PR, replacing with new PR

Old notes: https://github.com/dolphin-emu/dolphin/pull/1183
Globals should probably use the `g_` prefix, not the `m_` prefix? `m_byteBuffer` should probably be local to  `AVIDump::AddFrame(const u8*)` (its address could be replaced with `NULL` in `AVIDump::Stop`)