Allow middle-click scrolling

TODO.md

@@ -58,8 +58,6 @@ then switch back to the magnifier, the toolbar doesn't show any magnification hi
 * Can't glide thru tool options in Firefox, mobile Chrome;
 might be a pointer events spec interpretation issue, and it could easily be that the more technically correct browsers are where it's not working
 (Note: not a thing allowed by MS Paint)
-* Middle-click scrolling is prevented
-(Note: not a thing allowed by MS Paint)
 
 
 ### Menus

src/Selection.js

@@ -96,7 +96,7 @@ Selection.prototype.cut_out_background = function(){
 	var sel = this;
 	var cutout = sel.canvas;
 	if(transparency){
-		// @FIXME: this doesn't work well with transparency between 0 and 1
+		// @FIXME: this doesn't work well with nonbinary transparency
 		ctx.save();
 		ctx.globalCompositeOperation = "destination-out";
 		ctx.drawImage(cutout, sel.x, sel.y);

src/app.js

@@ -583,6 +583,9 @@ $app
 	){
 		return;
 	}
+	if(e.button === 1){
+		return; // allow middle-click scrolling
+	}
 	e.preventDefault();
 });
 

src/tools.js

@@ -473,6 +473,10 @@ tools = [{
 			ctx.fillRect(x, y, w, h);
 		}
 		if(this.$options.stroke){
+			// FIXME: can draw 1x2 or 2x1 pixels of a rectangle with a stroke of 1px (the default)
+			// which doesn't get drawn at full opacity
+			// or more generally, a 0-width or 0-height rectangle gives
+			// non-full-opacity pixels at either side of the resulting line drawn
 			if((stroke_size % 2) === 1){
 				ctx.strokeRect(x-0.5, y-0.5, w, h);
 			}else{