Make placement guide the correct size

src/js/drawtools.js

@@ -186,6 +186,8 @@ function enableGuideBox(image, scalecorrectionfactor) {
     var imageHeight = (image.height / (96 * scalecorrectionfactor)) * canvasCurrentDPI;
 
     var canvas = $("#placementguidebox")[0];
+    canvas.width = imageWidth;
+    canvas.height = imageHeight;
     var ctx = canvas.getContext("2d");
     ctx.drawImage(image, 0, 0, imageWidth, imageHeight);
 }