Fix PNGWriter exporting blank PNGs (#1807)

* Fix horrendous variable assignment in PNGWriter
 - Fixes #1679

* More detailed information on PNG format

* Avoid a larger number of blocks that do nto have a color assigned in TextureUtil

* Don't error in PNGWriter if biomes aren't saved to clipboard and grass is present

worldedit-core/src/main/java/com/sk89q/worldedit/extent/clipboard/io/BuiltInClipboardFormat.java

@@ -239,7 +239,8 @@ public enum BuiltInClipboardFormat implements ClipboardFormat {
     },
 
     /**
-     * Isometric PNG writer.
+     * Isometric PNG writer. Only supports solid, full-cube blocks and creates a view of the clipboard looking FROM the south
+     * east of the clipboard
      */
     PNG("png", "image") {
         @Override