Fix bug of clearing COF_TEMP_MARK. draw_shapes_in_dirty_areas() mark COF_TEMP_MARK on coords and clear them before leaving. But, it always clear the same one coord for several times and keep others untouched. It is a logical error. It can be avoided by avoiding to use the variable at mutliple place for different purpose in a function, and compiler will check it out by yelling that the variable is used before assigning a value.

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