Allow group member avatars to be passed an array of arguments, bringing them up-to-speed with pretty much all other avatar fetching functions. Pass the computed a`$r`guments array into the associated filters. (No changes to default arguments in these functions.)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8750 cdf35c40-ae34-48e0-9cc9-0c9da1808c22