GRAPHICS: Fix rounding error when using non-integral scaling

When a non-integral scaling was being used, x and/or y cursor position would be
one less than what it should be.

Fixes Trac#10401

Thanks snover!

backends/graphics/windowed.h

@@ -94,8 +94,8 @@ class WindowedGraphicsManager : virtual public GraphicsManager {
 			error("convertVirtualToWindow called without a valid draw rect");
 		}
 
-		return Common::Point(targetX + x * targetWidth / sourceWidth,
-							 targetY + y * targetHeight / sourceHeight);
+		return Common::Point(targetX + (x * targetWidth + sourceWidth / 2) / sourceWidth,
+		                     targetY + (y * targetHeight + sourceHeight / 2) / sourceHeight);
 	}
 
 	/**
@@ -120,8 +120,8 @@ class WindowedGraphicsManager : virtual public GraphicsManager {
 		x = CLIP<int>(x, sourceX, sourceMaxX);
 		y = CLIP<int>(y, sourceY, sourceMaxY);
 
-		return Common::Point(((x - sourceX) * targetWidth) / sourceWidth,
-							 ((y - sourceY) * targetHeight) / sourceHeight);
+		return Common::Point(((x - sourceX) * targetWidth + sourceWidth / 2) / sourceWidth,
+		                     ((y - sourceY) * targetHeight + sourceHeight / 2) / sourceHeight);
 	}
 
 	/**