Port over Bitmap 'radial_blur' from old SFML codebase

This implementation is also heaps better than the old
one as it doesn't use a (differently sized) aux texture,
meaning the Bitmap discards its old texture and aquires
one of same size, making reuse through the TexPool a
lot more likely. It also saves on the aux texture blits
and binding switches.

As the setup / resource acquisition far outweighs the
actual rendering cost, operation time is relatively
constant no matter how many divisions are used.

src/bitmap.h

@@ -74,7 +74,7 @@ public:
 	               int width, int height);
 	void clearRect(const IntRect &rect);
 
-//	void radialBlur(int angle, int divisions);
+	void radialBlur(int angle, int divisions);
 //	/* ----- */
 
 	void clear();