Added support for vertex shaders in sf::Shader

Rewrote the Shader example

examples/shader/resources/wave.vert

@@ -0,0 +1,15 @@
+uniform float wave_phase;
+uniform vec2 wave_amplitude;
+
+void main()
+{
+    vec4 vertex = gl_Vertex;
+    vertex.x += cos(gl_Vertex.y * 0.02 + wave_phase * 3.8) * wave_amplitude.x
+              + sin(gl_Vertex.y * 0.02 + wave_phase * 6.3) * wave_amplitude.x * 0.3;
+    vertex.y += sin(gl_Vertex.x * 0.02 + wave_phase * 2.4) * wave_amplitude.y
+              + cos(gl_Vertex.x * 0.02 + wave_phase * 5.2) * wave_amplitude.y * 0.3;
+
+	gl_Position = gl_ModelViewProjectionMatrix * vertex;
+	gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
+	gl_FrontColor = gl_Color;
+}