Iris Effect

I added the Iris effect for the scene changes. The effect is realized by adding a second bitmap layer which has priority over the bitmap displaying the actual scene. Initially the "effects" bitmap is filled with all pixels solid black. The view opens by drawing concentrical circles with increasing radius with transparent pixel color. The drawing routine itself is a variation of Bresenham's line algorithm, see https://en.wikipedia.org/wiki/Midpoint_circle_algorithm