Sprite: Add special case shader for translucent effect

Translucent sprites (opacity < 255) are very common,
so using a custom shader instead of the full blown effect one
helps a lot, especially on mobile.

shader/simpleAlphaUni.frag

@@ -0,0 +1,11 @@
+
+uniform sampler2D texture;
+uniform lowp float alpha;
+
+varying vec2 v_texCoord;
+
+void main()
+{
+	gl_FragColor = texture2D(texture, v_texCoord);
+	gl_FragColor.a *= alpha;
+}