macfb: fix 24-bit RGB pixel encoding

According to Apple Technical Note HW26: "Macintosh Quadra Built-In Video" the
in-built framebuffer encodes each 24-bit pixel into 4 bytes. Adjust the 24-bit
RGB pixel encoding accordingly which agrees with the encoding expected by MacOS
when changing into 24-bit colour mode.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20211007221253.29024-12-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/display/macfb.c

@@ -224,10 +224,10 @@ static void macfb_draw_line24(MacfbState *s, uint8_t *d, uint32_t addr,
     int x;
 
     for (x = 0; x < width; x++) {
-        r = macfb_read_byte(s, addr);
-        g = macfb_read_byte(s, addr + 1);
-        b = macfb_read_byte(s, addr + 2);
-        addr += 3;
+        r = macfb_read_byte(s, addr + 1);
+        g = macfb_read_byte(s, addr + 2);
+        b = macfb_read_byte(s, addr + 3);
+        addr += 4;
 
         *(uint32_t *)d = rgb_to_pixel32(r, g, b);
         d += 4;