Scaffolding for VGA CHAIN4 support

tests/pc/vga/L42-4.C

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+/* Function to draw a non-antialiased line from (X0,Y0) to (X1,Y1), using a
+ * simple fixed-point error accumulation approach.
+ * Tested with Borland C++ in C compilation mode and the small model.
+ */
+extern void DrawPixel(int, int, int);
+
+/* Non-antialiased line drawer.
+ * (X0,Y0),(X1,Y1) = line to draw, Color = color in which to draw
+ */
+void DrawLine(int X0, int Y0, int X1, int Y1, int Color)
+{
+	unsigned long ErrorAcc, ErrorAdj;
+	int DeltaX, DeltaY, XDir, Temp;
+
+	/* Make sure the line runs top to bottom */
+	if (Y0 > Y1) {
+		Temp = Y0; Y0 = Y1; Y1 = Temp;
+		Temp = X0; X0 = X1; X1 = Temp;
+	}
+	DrawPixel(X0, Y0, Color);  /* draw the initial pixel */
+	if ((DeltaX = X1 - X0) >= 0) {
+		XDir = 1;
+	} else {
+		XDir = -1;
+		DeltaX = -DeltaX; /* make DeltaX positive */
+	}
+	if ((DeltaY = Y1 - Y0) == 0)  /* done if only one point in the line */
+		if (DeltaX == 0) return;
+
+	ErrorAcc = 0x8000;	 /* initialize line error accumulator to .5, so we can
+						   advance when we get halfway to the next pixel */
+	/* Is this an X-major or Y-major line? */
+	if (DeltaY > DeltaX) {
+		/* Y-major line; calculate 16-bit fixed-point fractional part of a
+		 pixel that X advances each time Y advances 1 pixel */
+		ErrorAdj = ((((unsigned long)DeltaX << 17) / (unsigned long)DeltaY) + 1) >> 1;
+		/* Draw all pixels between the first and last */
+		do {
+			ErrorAcc += ErrorAdj;	   /* calculate error for this pixel */
+			if (ErrorAcc & ~0xFFFFL) {
+				/* The error accumulator turned over, so advance the X coord */
+				X0 += XDir;
+				ErrorAcc &= 0xFFFFL;	/* clear integer part of result */
+			}
+			Y0++;					   /* Y-major, so always advance Y */
+			DrawPixel(X0, Y0, Color);
+		} while (--DeltaY);
+		return;
+	}
+	/* It's an X-major line; calculate 16-bit fixed-point fractional part of a
+	  pixel that Y advances each time X advances 1 pixel */
+	ErrorAdj = ((((unsigned long)DeltaY << 17) / (unsigned long)DeltaX) + 1) >> 1;
+	/* Draw all remaining pixels */
+	do {
+		ErrorAcc += ErrorAdj;	   /* calculate error for this pixel */
+		if (ErrorAcc & ~0xFFFFL) {
+			/* The error accumulator turned over, so advance the Y coord */
+			Y0++;
+			ErrorAcc &= 0xFFFFL;	/* clear integer part of result */
+		}
+		X0 += XDir;				   /* X-major, so always advance X */
+		DrawPixel(X0, Y0, Color);
+	} while (--DeltaX);
+}