intrabc: use visible width/height in mv correction

Prevents adjusting intra block copy motion vectors to values pointing out of the current tile if the block is not entirely visible and is in one super block wide/high tile. Fixes an use of uninitilized value in inv_txfm_add_c() with clusterfuzz-testcase-minimized-dav1d_fuzzer_mt-5746740678885376. Credits to oss-fuzz.