Add SVGA multi-monitor support

Cirrus Logic CL-GD5436/CL-GD5446 works as secondary video adapters

src/include/86box/vid_svga.h

@@ -33,6 +33,8 @@
 #    define FLAG_RAMDAC_SHIFT 64
 #    define FLAG_128K_MASK    128
 
+struct monitor_t;
+
 typedef struct {
     int ena,
         x, y, xoff, yoff, cur_xsize, cur_ysize,
@@ -170,6 +172,12 @@ typedef struct svga_t {
     uint32_t (*remap_func)(struct svga_t *svga, uint32_t in_addr);
 
     void *ramdac, *clock_gen;
+
+    /* Monitor Index */
+    uint8_t monitor_index;
+
+    /* Pointer to monitor */
+    monitor_t* monitor;
 } svga_t;
 
 extern int vga_on, ibm8514_on;

src/include/86box/vid_voodoo_common.h

@@ -506,6 +506,7 @@ typedef struct voodoo_t {
     uint8_t *vram, *changedvram;
 
     void *p;
+    uint8_t monitor_index;
 } voodoo_t;
 
 typedef struct voodoo_set_t {