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;       

lib/OpenLayers/Layer/WMS.js

@@ -71,7 +71,7 @@ OpenLayers.Layer.WMS.prototype =
     },
 
     /**
-    * addTile creates a tile, initializes it (via 'draw' in this case), and 
+    * addTile creates a tile, initializes it, and 
     * adds it to the layer div. 
     *
     * @param {OpenLayers.Bounds} bounds

lib/OpenLayers/Tile/Image.js

@@ -36,9 +36,7 @@ OpenLayers.Tile.Image.prototype =
     /**
     */
     draw:function(transparent) {
-        if (false) { // don't actually use the alpha PNG hack right now
-                     // it has a fiercely bad effect on IE6's performance
-            // if (transparent) {
+        if (transparent) {
             this.imgDiv = OpenLayers.Util.createAlphaImageDiv(null,
                                                            this.position,
                                                            this.size,