Use glsl-unit in WebGL tile layer renderer

src/ol/renderer/webgl/webgltilelayer.glsl

@@ -0,0 +1,24 @@
+//! NAMESPACE=ol.renderer.webgl.tilelayer
+
+
+//! COMMON
+varying vec2 v_texCoord;
+
+
+//! VERTEX
+attribute vec2 a_position;
+attribute vec2 a_texCoord;
+uniform vec4 u_tileOffset;
+
+void main(void) {
+  gl_Position = vec4(a_position * u_tileOffset.xy + u_tileOffset.zw, 0., 1.);
+  v_texCoord = a_texCoord;
+}
+
+
+//! FRAGMENT
+uniform sampler2D u_texture;
+
+void main(void) {
+  gl_FragColor = texture2D(u_texture, v_texCoord);
+}