Prevent text selection within map canvas div to fix double-click issues.

See #6740

Double-clicking on the map breaks panning in Edge and IE11. The issue is
that after double-clicking, Edge & IE perform a text selection within
the div, so then subsequent drag actions are trying to drag the
selection instead of dragging the map. This fixes it by disabling text
selection within the container.

src/css/mapbox-gl.css

@@ -19,6 +19,10 @@
     cursor: -webkit-grab;
     cursor: -moz-grab;
     cursor: grab;
+    -moz-user-select: none;
+    -webkit-user-select: none;
+    -ms-user-select: none;
+    user-select: none;
 }
 
 .mapboxgl-canvas-container.mapboxgl-interactive:active,