Fix size computation for array textures in setStorage() fallback.
Fix size computation for array textures in setStorage() fallback.

The comment was right, but the code was apparently copy-pasted. Sadly no
unit test covered this, because nearly every desktop impl has
ARB_texture_storage and on ES2 (which is the only that doesn't have
texture storage) it's not possible to query image size, heh.

MiUishadow/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum,ashimidashajia/magnum,DerThorsten/magnum,DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,DerThorsten/magnum