riders: xmen colors for scroll A/B

[jotego]

scroll A/B colors are completely messed up. The fixed layer seems ok.

[jotego]

• It does not seem to be a problem with the color nibble order. Swapping them still looks wrong

    .lyra_pxl   ( { lyra_pxl[4+:4],lyra_pxl[11-:4],lyra_pxl[3:0]}  ),
    .lyrb_pxl   ( { lyrb_pxl[4+:4],lyrb_pxl[11-:4],lyrb_pxl[3:0]}  ),

• The lower four bits (directly coming from the ROM) are in the right order, as the right colors can be produced by introducing debug_bus into the color byte of the tiles.
• The problem can be reproduced in MAME by setting the color byte to zero for SCRA/SCRB:

K052109_CB_MEMBER(xmen_state::tile_callback)
{
    // (color & 0x02) is flip y handled internally by the 052109
    if (layer == 0)
        *color = m_layer_colorbase[layer] + ((*color & 0xf0) >> 4);
    else
        //*color = m_layer_colorbase[layer] + ((*color & 0x7c) >> 2);
        *color=m_layer_colorbase[layer]; // <- this reproduces the problem
}

Palette regions are assigned correctly:

• 6xx for fix
• 000 for SCRA
• 200 for SCRB

but the color bits are not filled correctly for SCRA/SCRB

[jotego]

The color mux MMR bit was not correct in the RE information. Reported to Furrtek and implemented on MAME (PR done). Fixed in 21e74c2f