Accept const ref Bitmap in createTextureFromBitmap

Neither does the function take shared ownership nor does it change the
reference. A texture is created from the data. Therefore, const ref is
fine (see GotW 91 solution 3 (b),
http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/).
Accept const ref Bitmap in TextRenderer

Neither does the class take shared ownership nor does it change the
reference. A texture is created from the data. Therefore, const ref is
fine (see GotW 91 solution 3 (b),
http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/).