servo: Merge #6239 - Correct the calculation of rects for drawimage (from hyowon:drawimage_rects); r=jdm

https://html.spec.whatwg.org/multipage/#dom-context-2d-drawimage
The source and destination rectangles have four points (x, y), (x+w, y), (x+w, y+h), (x, y+h), which doesn't mean rect(x, y, w, h).
cc @yichoi

Source-Repo: https://github.com/servo/servo
Source-Revision: 2a8d5952892e050a3d604741dd1007e3bd563315

servo/components/script/dom/canvasrenderingcontext2d.rs

@@ -140,8 +140,8 @@ impl CanvasRenderingContext2D {
 
         // The source rectangle is the rectangle whose corners are the four points (sx, sy),
         // (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).
-        let source_rect = Rect(Point2D(sx, sy),
-                               Size2D(sw, sh));
+        let source_rect = Rect(Point2D(sx.min(sx+sw), sy.min(sy+sh)),
+                               Size2D(sw.abs(), sh.abs()));
 
         // When the source rectangle is outside the source image,
         // the source rectangle must be clipped to the source image
@@ -158,8 +158,8 @@ impl CanvasRenderingContext2D {
 
         // The destination rectangle is the rectangle whose corners are the four points (dx, dy),
         // (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).
-        let dest_rect = Rect(Point2D(dx, dy),
-                             Size2D(dest_rect_width_scaled, dest_rect_height_scaled));
+        let dest_rect = Rect(Point2D(dx.min(dx+dest_rect_width_scaled), dy.min(dy+dest_rect_height_scaled)),
+                             Size2D(dest_rect_width_scaled.abs(), dest_rect_height_scaled.abs()));
 
         let source_rect = Rect(Point2D(source_rect_clipped.origin.x,
                                      source_rect_clipped.origin.y),