code: function startLine (e) {
              e.preventDefault();
  
              strokes = that.strokes;
              sketching = true;
              that.undos = [];
  
              var cursor = getCursorRelativeToCanvas(e);
              strokes.push({
                  points: [cursor],
                  color: opts.line.color,
                  size: getLineSizeRelativeToCanvas(opts.line.size),
                  cap: opts.line.cap,
                  join: opts.line.join,
                  miterLimit: opts.line.miterLimit
              });
          }
docs: On mouse down, create a new stroke with a start location