This patch implements the variable size mode of the blur node. This is
not identical to the CPU implementation, but is visually very close.

That's because of two things. First, the Extend Bounds option introduces
a 2px offset that doesn't make sense, which is likely a bug in the CPU
implementation. Second, the CPU implementation approximate the result
using three passes, the first two of which are separable morphological
operators applied on the size input. But this approximation does not
provide an advantage because the last pass is non-separable anyways. So
the GPU implementation does not attempt this approximation for more
accurate and faster results.