reset method
void
reset()
Implementation
void reset() {
_mScale = 1;
_mLastScale = 1;
_mScaleOffsetX = 0;
_mLastScaleOffsetX = 0;
_mScaleCenterPoint = Offset.zero;
_mPressOffset;
_mScrollX = 0;
_mExtentWidth = 0;
_mInnerWidth = 0;
}