Exposed filters form cache

Earl,

when i enable the "Exposed filters as a block" option, the form is processed two times: when the views is called and when hook_block is called (note that hook_block creates another instance of all views to check if they have the option enabled).

Its costy. And worse: it doesnt call hook_views_pre_view()!

This patch creates a small function that caches the form once its created.

regards,

massa