fix gaussianSelectiveBlur not a perfect circle

[lxlx5434]

Using GPUImageGaussianSelectiveBlurFilter results in an oval region but not a perfect circle: We can fix it by setting aspectRatio, but another issue occur. When I set excludeCirclePoint to (1.0, 1.0), I found that center of the circle not exactly at bottom-right: I fixed the shader string: Now it's a perfect circle and align any corner exactly.