style overrides: subtitle styling override

    This adds a mechanism that attempts to override dialogue lines, but not
    anything else, like typesetting.

    Requires the libass API user to request this explicitly. If this is not
    requested, libass will (or should) behave like without this commit.

    mpv patch included as 0001-sub-add-ass-style-override-force-option.patch
    (read the commit message)