Remove hard-coded 256px tile dimensions from providers

The proxy provider had a hard coded check for 256x256 tile request prior to requesting the tile.  If you were requesting a 512x512 tile from a remote source, the provider would raise an exception regardless of what size tile the remote source returned.  Removed the 256 limitation on proxy provider.  Mismatched image size is still handled correctly later in the stack.

In Sandwich provider, a local bitmap (non-layer) was hard coded to 256 instead of using the tile_height of the respective layer.  local_bitmap now takes dimension as a 4th argument, representing the size of the tile.