Improved H.264 and subtitle support

After some extensive tests streaming H.264 (MKV and MP4) videos with my Sony Bravia EX525, I decided to share my modifications to original code. I don't know if these modifications will break compatibility with other renderers (for example, I disabled mod 4 resolution constraint check for all Bravia TVs, mine worked fine with movies not respecting this particular requirement).

MEncoder encoding performance was terribly poor (4-year-old Phenom II X4 P920 with ATI Mobility Radeon HD 5650), but performance FFmpeg performed very well, although it was very difficult to fill the buffer to 100%, leading to 100% CPU usage almost all time (not a problem to me).

Also added better suport to subtitle encoding change (e.g.: from Windows-1252 to UTF-8) when ASS subtitles are enabled with FFmpeg.