fix: 修复导出的PNG位置不居中和图片错位的问题 (#23)

* fix: 修复导出的PNG位置不居中和图片错位的问题

* fix(png): fix bug more elegant.

* fix(png): rollback to former status

thanks to @qingyang912

* fix(png): syntax error

src/protocol/png.js

@@ -101,8 +101,8 @@ define(function(require, exports, module) {
                     url: imageUrl,
                     width: imageSize.width,
                     height: imageSize.height,
-                    x: -renderContainer.getBoundaryBox().x + imageRenderBox.x + 20,
-                    y: -renderContainer.getBoundaryBox().y + imageRenderBox.y + 20
+                    x: -renderContainer.getBoundaryBox().x + imageRenderBox.x,
+                    y: -renderContainer.getBoundaryBox().y + imageRenderBox.y
                 };
 
                 imagesInfo.push(imageInfo);
@@ -158,9 +158,9 @@ define(function(require, exports, module) {
 
         function drawImage(ctx, image, x, y, width, height) {
             if (width && height) {
-                ctx.drawImage(image, x, y, width, height);
+                ctx.drawImage(image, x + padding, y + padding, width, height);
             } else {
-                ctx.drawImage(image, x, y);
+                ctx.drawImage(image, x + padding, y + padding);
             }
         }
 
@@ -185,7 +185,7 @@ define(function(require, exports, module) {
                 return loadImages(imagesInfo);
             }).then(function($images) {
                 for(var i = 0; i < $images.length; i++) {
-                    drawImage(ctx, $images[i].element, $images[i].x, $images[i].y, $images[i].width, $images[i].height);
+                    drawImage(ctx, $images[i].element, $images[i].x + offsetX, $images[i].y + offsetY, $images[i].width, $images[i].height);
                 }
 
                 DomURL.revokeObjectURL(svgDataUrl);