intrabc: use visible width/height in mv correction
Prevents adjusting intra block copy motion vectors to values pointing out of the current tile. This happens with not entirely visible blocks in a 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.
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