eina_tiler: use int size matching Eina_Rectangle internally to avoid overflow

eina_tiler: use int size matching Eina_Rectangle internally to avoid overflow

since this code's creation it seems that the internal int size was set to use
short in order to micro-optimize memory usage, while the api function parameters
used Eina_Rectangle which had a larger int size. when initializing the internal
rect struct, this would lead to overflows which resulted in broken tilers which
returned iterators with no valid rects after having valid rects added

test case: run weston-subsurfaces

@fix