This improves readability a lot and prevent us from reinventing the wheel.

    use size_t instead of int to represent sizes
    removes unused parameters from:
        block_BytestreamInit()
        mpeg4_audio/LOASSyncInfo()
        cinepak/cinepak_Getv1()
        speex/SendPacket()
        subsass/ParseColor()
        ffmpeg/postprocess/InitPostproc() & PostprocPict()
        dvbsub/YuvaYuvp() & encode_pixel_line_{2,4,8}bp()
        faad/DoReordering()

vlc_block_helper.h:
    use size_t, removes unused parameter from block_BytestreamInit()

struct filter_t:
    removes unused parameters from pf_picture_link() & pf_picture_unlink()
    
cmml: fix a segfault (p_item isn't an input item, despite the name) present for some months, proving that nobody did use that code ?

The rollup and paint on mode is not really good (a lot of flicker) but
I don't see how to fix it (the subtitle is updated as soon as a new
character is added).