Vast fixes to the Mach8/32/8514/A compatible side (April 22nd 2025)

1. As tiring as it seems, XOR properly the passthrough bits for mode changes. Fixes everything tested as of now (Mach8/32).
2. Implemented separate destination and source GE pitch and offsets. Fixes Windows 3.1 mach8/32 drivers using devicebitmap enabled (or dmp=1).
3. Properly fixed high and true color patterns so that the aforementioned patterns are drawn correctly in various stuff.
4. Implemented in the best possible way the CRT offset (currently in on conjunction with the GE offset).
5. On ScanToX with bpp set to 24 and dpconfig 0x6211, optmize the SRC/CUR X-Y coordinates at the end of the blit.
6. For mode changes, don't call the mode change recalctimings function if we aren't in the appropriate bits of the respective ports.
7. Separate 8514/A compatible and Extended foreground/background selection implemented, fixes Windows 3.x ATI Ultra drivers from 1992 without hacks.
8. Clear the busy flags when needed without stalling the entire guests (but not the emulator itself) (Mach8/32).
9. The MMIO regs should be enabled only when the VGA aperture has reached a graphics mode (0xA000). This fixes NT 3.x mach drivers in true color mode and making Win3.1 drivers working without a hitch at the same time.
10. Actually implement the hardware 4bpp cursor properly on Mach32.
11. Cosmetic cleanups.

src/include/86box/vid_svga.h

@@ -171,11 +171,11 @@ typedef struct svga_t {
     latch_t  latch;
 
     pc_timer_t timer;
-    pc_timer_t timer8514;
+    pc_timer_t timer_8514;
     pc_timer_t timer_xga;
 
     double clock;
-    double clock8514;
+    double clock_8514;
     double clock_xga;
 
     double multiplier;