Mixin for tiles that use form-encoded POST requests to get images from remote services.  Images will be loaded using HTTP-POST into an IFrame.

This mixin will be applied to OpenLayers.Tile.Image instances configured with OpenLayers.Tile.Image.maxGetUrlLength set.

Instances of OpenLayers.Tile.Image are used to manage the image tiles used by various layers.
{Number} If set, requests that would result in GET urls with more characters than the number provided will be made using form-encoded HTTP POST.