Fix breakage in r4694

[TomH, 2012-07-20 19:06:24]

Move the reset back before setting up the custom stage.
Also, this is one of the (few?) places we're still getting textures passed
around in fTextures, and so need to do some bookkeeping to satisfy
sanity-checking asserts later on.
We could set fTextures[0] to the new filtered texture, or to NULL.

[TomH, 2012-07-20 19:15:28]

Committed in r4700 after verbal LGTM by bsalomon@.