fix renderer bug when displaying a circle filled in with rgba color

Created by: robbycerantola

@jackpot51 There is a bug rendering a circle filled with rgba color when alfa is not ff ; there are some horizontal line because those pixel are cycled multiple times .

I pushed a fix so the circle will render corectly when filled in rgba colors.

Updated example , resolved some complains in sdl2.rs about "variable does not need to be mutable" too.

Hope you like it.