Cirrus, ATI Mach8/32 and XGA fixes.

1. Update svga->fast to account for packed chain mode toggles, fixes issues on Descent II for DOS using the Cirrus cards.
2. Re-organized ATI Mach8/32 LFB access as well as 8514/A/ATI mode toggles, should end the video mode issues once and for all.
3. Fixed a small but major pattern issue with IBM OS/2 1.30.1's XGA driver (not .2, which is fine as is).

src/include/86box/vid_8514a.h

@@ -137,7 +137,7 @@ typedef struct ibm8514_t {
     } accel;
 
     uint16_t test;
-    int      ibm_mode;
+    int      vendor_mode[2];
 
     int      v_total;
     int      dispend;