mapscript selectoutputformat should clear the rendering caches if the driver is changed

[tbonfort]

this bug isn't active yet. It would occur with the plugin renderers, if subsequent drawing functions are called and the outputformat has changed.

• create a mapserver function that clears the renderer caches (TODO: tbonfort)

• have mapscript call that function if the outputformat is changed

[aboudreault]

When the function is created, I'll take care of the mapscript calls.

[tbonfort]

waiting to see if mapscript users get bitten by this before investing too much time into it: pushing to FUTURE.