Fix some "signed shift" warnings

[kevinbackhouse]

Fix some more "signed shift" warnings flagged by CodeQL. The previous batch was in #2266.

[codecov]

Codecov Report

Merging #2271 (7f673c7) into main (a5c521e) will not change coverage.
The diff coverage is 93.10%.

@@           Coverage Diff           @@
##             main    #2271   +/-   ##
=======================================
  Coverage   63.46%   63.46%           
=======================================
  Files         118      118           
  Lines       19598    19598           
  Branches     9556     9556           
=======================================
  Hits        12438    12438           
  Misses       5094     5094           
  Partials     2066     2066           

Impacted Files	Coverage Δ
src/webpimage.cpp	42.66% <0.00%> (ø)
app/exiv2.cpp	62.55% <100.00%> (ø)
src/canonmn_int.cpp	71.55% <100.00%> (ø)
src/convert.cpp	51.40% <100.00%> (ø)
src/image.cpp	69.57% <100.00%> (ø)
src/nikonmn_int.cpp	57.10% <100.00%> (ø)
src/pentaxmn_int.cpp	71.49% <100.00%> (ø)
src/pngchunk_int.cpp	74.12% <100.00%> (ø)

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