build: add -Werror=format-security, add -W flags in all platforms

The idea of using -Werror=format-security comes from MPlayer.

Try to use the compiler flags with any compiler. There's no reason not
to apply them on clang.
vo_opengl: don't assume legacy OpenGL is available with GLES

OpenGL 3.0 has the weird situation that it's "hard" to detect whether
legacy GL is really present. Since we pretend that we have OpenGL 3.0 if
we use GLES, it was assuming legacy GL is available.
command: extend revert_seek command

"revert_seek mark" basically forces the seekback point. It's basically a
one-way bookmark.
player: cosmetics: remove ancient comment

The DVD horror was confined to specific parts of the player, instead of
having it spread everywhere.