Fix 3D cube icon block item texture regression

Block textures are 3D cubes by default, even if only one texture name
was specified (as a string). We used to handle this situation by
toarray()ing the name before resolving the texture URLs, which
translated str -> [str], but now that {top,left,front} objects are
supported for texture names, we instead check manually. Fixes most
blocks rendering as flat textures in inventory windows in 0.8.0.
0.8.1