Changes

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The problem arose when I put multiple instances of XObject in design. Due to the static keyword, this variable would be shared among all of the animated objects. The first animation would run smoothly, while any animations added afterwords would be extremely slow. This is because this variable tells the function when the last time it drew the object's state along the animation time-line, and each object's time-line is different. To fix this, I made last time a data member and initialized it to 0 in the constructor.