fix raster tile flickering

fixes #1270

Use the depth buffer to prevent double drawing in areas where raster
tiles overlap.

js/render/draw_raster.js

@@ -5,13 +5,21 @@ var util = require('../util/util');
 module.exports = drawRaster;
 
 function drawRaster(painter, source, layer, coords) {
-    for (var i = 0; i < coords.length; i++) {
+    if (painter.isOpaquePass) return;
+
+    var gl = painter.gl;
+
+    // Change depth function to prevent double drawing in areas where tiles overlap.
+    gl.depthFunc(gl.LESS);
+
+    for (var i = coords.length - 1; i >= 0; i--) {
         drawRasterTile(painter, source, layer, coords[i]);
     }
+
+    gl.depthFunc(gl.LEQUAL);
 }
 
 function drawRasterTile(painter, source, layer, coord) {
-    if (painter.isOpaquePass) return;
 
     painter.setDepthSublayer(0);