This is cleaner than the if statements in the code. Also np2 textures
should in theory support linear filtering, but fglrx doesn't seem to
like it. This needs further investigation. So far we've never used
linear filtering on np2 textures, so there should not be a
regression. Furthermore I think shader support is more important than
filtering, since NP2 textures are mostly used for 1:1 copying to the
screen.