Sierra RAMDAC now correctly ignores writes of 0xFF to the command register;

BT485 dummy code now passes reads and writes to the generic SVGA handler;
Minor S3 changes;
ATI Mach64/GX now has an 8 MB RAM option.

src/vid_ati_mach64.c

@@ -2706,6 +2706,10 @@ static device_config_t mach64gx_config[] =
                                 .description = "4 MB",
                                 .value = 4
                         },
+                        {
+                                .description = "8 MB",
+                                .value = 8
+                        },
                         {
                                 .description = ""
                         }