Added support for vertex shaders in sf::Shader

Rewrote the Shader example

examples/shader/resources/pixelate.frag

@@ -0,0 +1,9 @@
+uniform sampler2D texture;
+uniform float pixel_threshold;
+
+void main()
+{
+    float factor = 1.0 / (pixel_threshold + 0.001);
+	vec2 pos = floor(gl_TexCoord[0].xy * factor + 0.5) / factor;
+	gl_FragColor = texture2D(texture, pos) * gl_Color;	
+}