fix: build break from untested combo of two PRs (#4794)

Oops, my bad. Looks like #4664 (merged about a week ago) and #4783
(merged today, but authored before #4664 was merged) interacted in a way
that produced a build break. This tiny fix is essential to unbreak it.

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

src/libtexture/imagecache.cpp

@@ -3915,7 +3915,7 @@ ImageCacheImpl::add_tile(ustring filename, int subimage, int miplevel, int x,
     }
     if (chend < chbegin) {  // chend < chbegin means "all channels."
         chbegin = 0;
-        chend   = file->spec(subimage, miplevel).nchannels;
+        chend   = file->spec(subimage).nchannels;
     }
     TileID tileid(*file, subimage, miplevel, x, y, z, chbegin, chend);
     ImageCacheTileRef tile