ATI Pre-Mach64 (Mach8, Mach32) changes

Satisfy refresh rates correctly on said chips and add the clock chip of the add-on 8514/A Ultra.

src/include/86box/vid_8514a.h

@@ -253,6 +253,7 @@ typedef struct ibm8514_t {
     int     vdisp;
     int     vdisp2;
     int     disp_cntl;
+    int     disp_cntl_2;
     int     interlace;
     uint16_t subsys_cntl;
     uint8_t subsys_stat;

src/include/86box/vid_ati_mach8.h

@@ -109,6 +109,7 @@ typedef struct mach_t {
         uint16_t src_y;
         int16_t  bres_count;
         uint16_t clock_sel;
+        uint16_t clock_sel_mode;
         uint16_t crt_pitch;
         uint16_t ge_pitch;
         uint16_t src_pitch;

src/include/86box/vid_svga.h

@@ -140,6 +140,7 @@ typedef struct svga_t {
     int vblankend;
     int render_line_offset;
     int start_retrace_latch;
+    int vga_mode;
 
     /*The three variables below allow us to implement memory maps like that seen on a 1MB Trio64 :
       0MB-1MB - VRAM