Added an 'alpha' option to Layer.Grid by which Layers can have the PNG alpha hack enabled on a per-Layer basis.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@840 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Layer/Grid.js

@@ -19,6 +19,10 @@ OpenLayers.Layer.Grid.prototype = Object.extend( new OpenLayers.Layer(), {
     // this is an array of rows, each row is an array of tiles
     grid: null,
 
+    // alpha: boolean
+    // asserts whether or not the layer's images have an alpha channel
+    alpha: false,
+
     /**
     * @param {str} name
     * @param {str} url
@@ -161,7 +165,7 @@ OpenLayers.Layer.Grid.prototype = Object.extend( new OpenLayers.Layer(), {
                                         new OpenLayers.Pixel(tileoffsetx - parseInt(this.map.layerContainerDiv.style.left),
                                                              tileoffsety - parseInt(this.map.layerContainerDiv.style.top))
                                                             );
-                tile.draw((this.params.TRANSPARENT == 'true'));
+                tile.draw(this.alpha);
                 row.append(tile);
      
                 tileoffsetlon += tilelon;