evas filters: Switch to uniforms in GL blur

evas filters: Switch to uniforms in GL blur

This was a poor attempt at improving the performance but
obviously the root cause isn't fixed (too many texel fetches).

Uniform should (theoretically) work better than an attribute
the for loop. Just a guess here.

This also makes GL blur use a float value as radius, allowing
future extension to non-integer blur radii, as well as using
linear scaling as a fast blur approximation.