Fix seek bugs.

• Fix seek bug that could block the player when seeking backward.
• Use lexical_cast instead of std::atoi in subtitle parsing.

Subtitle support

• Add ass & srt subtitle support. You can press s to switch subtitle track embedded in your media file or specify a srt file using --srt on the command line.
• Fix curl downloading the whole file. Only a certain amount of data is now downloaded at the time.
• Add freetype2 text renderer.
• Fix audio channel mapping
• Add icon on windows.
• Fix windows argument parsing on the shell and when called from the explorer when a file type is associated to the player.