Added Matrox MGA (Mystique) emulation from PCem.

src/video/vid_svga.h

@@ -109,6 +109,11 @@ typedef struct svga_t
     void (*ven_write)(struct svga_t *svga, uint8_t val, uint32_t addr);
     float (*getclock)(int clock, void *p);
 
+	/*Called when VC=R18 and friends. If this returns zero then MA resetting
+	  is skipped. Matrox Mystique in Power mode reuses this counter for
+	  vertical line interrupt*/
+	int (*line_compare)(struct svga_t *svga);    
+
     /*If set then another device is driving the monitor output and the SVGA
       card should not attempt to display anything */
     int override;