Fix problem on fabric.Line controls and line.getWidth() (#3742)

* feat(pr/fixLineWidth) Fix problem on fabric.Line controls and line.getWidth()

* fix line boxes and svg import

src/shapes/line.class.js

@@ -167,10 +167,11 @@
         //  Line coords are distances from left-top of canvas to origin of line.
         //  To render line in a path-group, we need to translate them to
         //  distances from center of path-group to center of line.
-        var cp = this.getCenterPoint();
+        var cp = this.getCenterPoint(),
+            offset = this.strokeWidth / 2;
         ctx.translate(
-          cp.x - this.strokeWidth / 2,
-          cp.y - this.strokeWidth / 2
+          cp.x - (this.strokeLineCap === 'butt' && this.height === 0 ? 0 : offset),
+          cp.y - (this.strokeLineCap === 'butt' && this.width === 0 ? 0 : offset)
         );
       }
 
@@ -222,10 +223,10 @@
     _getNonTransformedDimensions: function() {
       var dim = this.callSuper('_getNonTransformedDimensions');
       if (this.strokeLineCap === 'butt') {
-        if (dim.x === 0) {
+        if (this.width === 0) {
           dim.y -= this.strokeWidth;
         }
-        if (dim.y === 0) {
+        if (this.height === 0) {
           dim.x -= this.strokeWidth;
         }
       }