WebP codec: turn exact mode when creating lossless files to avoid altering...

Even Rouaultrequested to merge
rouault/libtiff:webp_lossless_exact into master

WebP codec: turn exact mode when creating lossless files to avoid altering R,G,B values in areas where alpha=0

Fixes https://github.com/OSGeo/gdal/issues/8038

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