enum {
    ...
    RASTER_SHIFT2_R = ((4-RASTER_SHIFT_R)*2),
    RASTER_SHIFT2_G = ((4-RASTER_SHIFT_G)*2),
    RASTER_SHIFT2_B = ((4-RASTER_SHIFT_B)*2),
};

r = (r<<RASTER_SHIFT_R) + (r>>RASTER_SHIFT2_R);
g = (g<<RASTER_SHIFT_G) + (g>>RASTER_SHIFT2_G);
b = (b<<RASTER_SHIFT_B) + (b>>RASTER_SHIFT2_B);