[win32] Choose best Image handle drawing with GC

This commit adapts the drawImage method in GC in the windows
implementation for copying and (probably) scaling an area from a source
image into the GC target. With the Image supporting multiple handles now,
the handle from the source image is no longer chosen by the zoom context
of the GC but from the target size, e.g. if an image should be drawn
with a scale of 2 on a 100% monitor, it might provide better results to
use an image handle fitting for 200%.

Author: akoch-yatta

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/GC.java

@@ -985,31 +985,61 @@ public void drawImage (Image image, int srcX, int srcY, int srcWidth, int srcHei
 	if (image == null) SWT.error (SWT.ERROR_NULL_ARGUMENT);
 	if (image.isDisposed()) SWT.error(SWT.ERROR_INVALID_ARGUMENT);
 
-	int imageZoom = getZoom();
-	Rectangle src = DPIUtil.scaleUp(drawable, new Rectangle(srcX, srcY, srcWidth, srcHeight), imageZoom);
+	int gcZoom = getZoom();
+	int srcImageZoom = calculateZoomForImage(gcZoom, srcWidth, srcHeight, destWidth, destHeight);
+	drawImage(image, srcX, srcY, srcWidth, srcHeight, destX, destY, destWidth, destHeight, gcZoom, srcImageZoom);
+}
+
+private int calculateZoomForImage(int gcZoom, int srcWidth, int srcHeight, int destWidth, int destHeight) {
+	if (srcWidth == 1 && srcHeight == 1) {
+		// One pixel images can use the GC zoom
+		return gcZoom;
+	}
+	if (destWidth == srcWidth && destHeight == srcHeight) {
+		// unscaled images can use the GC zoom
+		return gcZoom;
+	}
+	if (gcZoom > 175) {
+		// big images can use the GC zoom
+		return gcZoom;
+	}
+
+	float imageScaleFactor = 1f*destWidth/srcWidth;
+	if (gcZoom*imageScaleFactor > 175) {
+		return 200;
+	}
+	return gcZoom;
+}
+
+private void drawImage(Image image, int srcX, int srcY, int srcWidth, int srcHeight, int destX, int destY,
+		int destWidth, int destHeight, int imageZoom, int scaledImageZoom) {
+	Rectangle src = DPIUtil.scaleUp(drawable, new Rectangle(srcX, srcY, srcWidth, srcHeight), scaledImageZoom);
 	Rectangle dest = DPIUtil.scaleUp(drawable, new Rectangle(destX, destY, destWidth, destHeight), imageZoom);
-	if (imageZoom != 100) {
+	if (scaledImageZoom != 100) {
 		/*
 		 * This is a HACK! Due to rounding errors at fractional scale factors,
 		 * the coordinates may be slightly off. The workaround is to restrict
 		 * coordinates to the allowed bounds.
 		 */
-		Rectangle b = image.getBounds(imageZoom);
+		Rectangle b = image.getBounds(scaledImageZoom);
 		int errX = src.x + src.width - b.width;
 		int errY = src.y + src.height - b.height;
 		if (errX != 0 || errY != 0) {
-			if (errX <= imageZoom / 100 && errY <= imageZoom / 100) {
+			if (errX <= scaledImageZoom / 100 && errY <= scaledImageZoom / 100) {
 				src.intersect(b);
 			} else {
 				SWT.error (SWT.ERROR_INVALID_ARGUMENT);
 			}
 		}
 	}
-	drawImage(image, src.x, src.y, src.width, src.height, dest.x, dest.y, dest.width, dest.height, false);
+	drawImage(image, src.x, src.y, src.width, src.height, dest.x, dest.y, dest.width, dest.height, false, scaledImageZoom);
 }
 
 void drawImage(Image srcImage, int srcX, int srcY, int srcWidth, int srcHeight, int destX, int destY, int destWidth, int destHeight, boolean simple) {
-	int imageZoom = getZoom();
+	drawImage(srcImage, srcX, srcY, srcWidth, srcHeight, destX, destY, destWidth, destHeight, simple, getZoom());
+}
+
+void drawImage(Image srcImage, int srcX, int srcY, int srcWidth, int srcHeight, int destX, int destY, int destWidth, int destHeight, boolean simple, int imageZoom) {
 	if (data.gdipGraphics != 0) {
 		//TODO - cache bitmap
 		long [] gdipImage = srcImage.createGdipImage(imageZoom);