private void expandDirtyRect(float historicalX, float historicalY) {
        if (historicalX < mDirtyRect.left) {
            mDirtyRect.left = historicalX;
        } else if (historicalX > mDirtyRect.right) {
            mDirtyRect.right = historicalX;
        }
        if (historicalY < mDirtyRect.top) {
            mDirtyRect.top = historicalY;
        } else if (historicalY > mDirtyRect.bottom) {
            mDirtyRect.bottom = historicalY;
        }
    }
Called when replaying history to ensure the dirty region includes all
mPoints.

@param historicalX the previous x coordinate.
@param historicalY the previous y coordinate.
signature-pad/src/main/java/com/github/gcacace/signaturepad/views/SignaturePad.java