sfRenderStates "constructors" from shaders and textures (issue #20)

The set of constructors in the C++ API is conflated into a create function
with all possible default values in the parameters. Documentation inspired
by the C++ version.

Also, a default function is declared reusing those defaults in order to
match the following change made in CSFML:
SFML/CSFML#178

include/sf-graphics-renderstates.ads

@@ -38,6 +38,43 @@ package Sf.Graphics.RenderStates is
    end record;
    type sfRenderStates_Ptr is access all sfRenderStates;
 
+
+   --//////////////////////////////////////////////////////////
+   --/ @brief Construct a set of render states with all its attributes
+   --/
+   --/ @param blendMode Blend mode to use
+   --/ @param transform Transform to use
+   --/ @param texture   Texture to use
+   --/ @param shader    Shader to use
+   --/
+   --/ If you want to use a single specific render state,
+   --/ for example a shader, you can call create with only one of the parameters,
+   --/ and then pass the result to the draw subprogram.
+   --//////////////////////////////////////////////////////////
+   function create
+     (BlendMode : Sf.Graphics.BlendMode.sfBlendMode := Sf.Graphics.BlendMode.sfBlendAlpha;
+      Transform : Sf.Graphics.Transform.sfTransform := Sf.Graphics.Transform.Identity;
+      Texture : sfTexture_Ptr := null;
+      Shader : sfShader_Ptr := null) return sfRenderStates
+     is (blendMode => blendMode,
+         transform => transform,
+         texture => texture,
+         shader => shader);
+
+   --//////////////////////////////////////////////////////////
+   --/ @brief Construct a default set of render states
+   --/
+   --/ Constructing a default set of render states is equivalent
+   --/ to calling create without parameters.
+   --/ The default set defines:
+   --/ - the BlendAlpha blend mode
+   --/ - the identity transform
+   --/ - a null texture
+   --/ - a null shader
+   --/
+   --//////////////////////////////////////////////////////////
+   function default return sfRenderStates is (create);
+
 private
    pragma Convention (C_Pass_By_Copy, sfRenderStates);
    pragma Convention (C, sfRenderStates_Ptr);