Fix projection texture painting crash

It was caused by own mistake by not noticing externtex is used not
only by render engine. Now this function uses pool passed as argument
rather than using R.pool.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53984 954f8c5b-7b00-dc11-b283-0030488c597c