S3 XGA: Take COLOR_CMP into consideration if asked by guest OS, wheth…

…er or not to draw the pixel to the destination. Preliminary testing shows this allows one Windows 95 game, using DirectX, to draw the 256-color UI bitmaps properly in which one color is the designated transparent color key

src/hardware/vga_xga.cpp

@@ -166,6 +166,21 @@ void XGA_DrawPoint(Bitu x, Bitu y, Bitu c) {
 
 }
 
+Bitu XGA_PointMask() {
+	switch(XGA_COLOR_MODE) {
+		case M_LIN8:
+			return 0xFFul;
+		case M_LIN15:
+		case M_LIN16:
+			return 0xFFFFul;
+		case M_LIN32:
+			return 0xFFFFFFFFul;
+		default:
+			break;
+	}
+	return 0;
+}
+
 Bitu XGA_GetPoint(Bitu x, Bitu y) {
 	uint32_t memaddr = (uint32_t)((y * XGA_SCREEN_WIDTH) + x);
 
@@ -735,6 +750,7 @@ void XGA_BlitRect(Bitu val) {
 	uint32_t xat, yat;
 	Bitu srcdata;
 	Bitu dstdata;
+	Bitu colorcmpdata;
 
 	Bitu srcval;
 	Bitu destval;
@@ -747,6 +763,8 @@ void XGA_BlitRect(Bitu val) {
 	if(((val >> 5) & 0x01) != 0) dx = 1;
 	if(((val >> 7) & 0x01) != 0) dy = 1;
 
+	colorcmpdata = xga.color_compare & XGA_PointMask();
+
 	srcx = xga.curx;
 	srcy = xga.cury;
 	tarx = xga.destx;
@@ -812,10 +830,21 @@ void XGA_BlitRect(Bitu val) {
 					break;
 			}
 
-			destval = XGA_GetMixResult(mixmode, srcval, dstdata);
-			//LOG_MSG("XGA: DrawPattern: Mixmode: %x Mixselect: %x", mixmode, mixselect);
+			bool doit = true;
 
-			XGA_DrawPoint((Bitu)tarx, (Bitu)tary, destval);
+			if (xga.control1 & 0x100) { /* COLOR_CMP enabled. control1 corresponds to XGA register BEE8h */
+				/* control1 bit 7 is SRC_NE.
+				 * If clear, don't update if source value == COLOR_CMP.
+				 * If set, don't update if source value != COLOR_CMP */
+				doit = !!(((srcval == colorcmpdata)?0:1)^((xga.control1>>7u)&1u));
+			}
+
+			if (doit) {
+				destval = XGA_GetMixResult(mixmode, srcval, dstdata);
+				//LOG_MSG("XGA: DrawPattern: Mixmode: %x Mixselect: %x", mixmode, mixselect);
+
+				XGA_DrawPoint((Bitu)tarx, (Bitu)tary, destval);
+			}
 
 			srcx += dx;
 			tarx += dx;