Fix T51678: 16bit greyscale PNG images thumbnailing fails

generate_local_thumbnails() now uses pathlib and f-string formatting too,
making the code a lot simpler. Furthermore, I removed unused bits of
resize_and_crop() and simplified the rest.

tests/test_api/images/README.txt

@@ -0,0 +1,2 @@
+Images courtesy of Blender Cloud
+https://cloud.blender.org/