Use MAKE_ACCESSORS() to generate accessors for 4 bpp RGB formats

Use MAKE_ACCESSORS() to generate accessors for 4 bpp RGB formats

Use FETCH_4 and STORE_4 macros to add support for 4bpp pixels to
fetch_and_convert_pixel() and convert_and_store_pixel(), then use
MAKE_ACCESSORS() to generate accessors for 4 bpp formats, except g4 and
c4 which are indexed:

    a4
    r1g2b1
    b1g2r1
    a1r1g1b1
    a1b1g1r1