VGA: clip display end against vtotal (bugfix for "Monolith" by TSI).

Part 4 of the demo allows you to select a nonstandard 320x570 to display
an image. Their routine however sets up the mode by programming
vtotal=594 vdend=1024 vblank=1024-1105 and vretrace=511-527, which
confuses DOSBox's calculations on active display area. It also causes
the demo to hang because the demo waits for vertical retrace, which no
longer functions because of the confusion. This fix limits the display
end against vtotal so the demo's strange VGA mode can work correctly.

src/hardware/vga_draw.cpp

@@ -1718,6 +1718,13 @@ void VGA_SetupDrawing(Bitu /*val*/) {
 		fps=(double)clock/(vtotal*htotal);
 		LOG(LOG_VGA,LOG_NORMAL)("VGA refresh rate is now, %.3f",vga_force_refresh_rate);
 	}
+
+	/* clip display end to stay within vtotal ("Monolith" demo part 4 320x570 mode fix) */
+	if (vdend > vtotal) {
+		LOG(LOG_VGA,LOG_WARN)("VGA display end greater than vtotal!");
+		vdend = vtotal;
+	}
+
 	// Horizontal total (that's how long a line takes with whistles and bells)
 	vga.draw.delay.htotal = htotal*1000.0/clock; //in milliseconds
 	// Start and End of horizontal blanking