Properly size circular buffer

In most cases, it should now use less memory and be faster to construct.
In the other cases, blurring used to be incorrect because the buffer was too small.

src/sdl/utils.cpp

@@ -1603,7 +1603,7 @@ surface blur_alpha_surface(const surface &surf, int depth)
 		depth = max_blur;
 	}
 
-	boost::circular_buffer<Uint32> queue(max_blur);
+	boost::circular_buffer<Uint32> queue(depth*2+1);
 
 	const Uint32 ff = 0xff;