Added support for vertex shaders in sf::Shader

Rewrote the Shader example

examples/shader/resources/storm.vert

@@ -0,0 +1,19 @@
+uniform vec2 storm_position;
+uniform float storm_total_radius;
+uniform float storm_inner_radius;
+
+void main()
+{
+    vec4 vertex = gl_ModelViewMatrix * gl_Vertex;
+    vec2 offset = vertex.xy - storm_position;
+    float len = length(offset);
+    if (len < storm_total_radius)
+    {
+        float push_distance = storm_inner_radius + len / storm_total_radius * (storm_total_radius - storm_inner_radius);
+        vertex.xy = storm_position + normalize(offset) * push_distance;
+    }
+
+	gl_Position = gl_ProjectionMatrix * vertex;
+	gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
+	gl_FrontColor = gl_Color;
+}