Add layer generation to vc_render

[csparker247]

This adds the layer generation functionality of vc_layers into vc_render. Simply set the rendering method to 4 on the command line:

# saves to ./20231122120123_#.tif
vc_render -v Foo.volpkg -s 20231122120123 --method 4

# saves to layers/20231122120123_#.tif
vc_render -v Foo.volpkg -s 20231122120123 --method 4 -o layers/

# saves to layers/#.tif
vc_render -v Foo.volpkg -s 20231122120123 --method 4 -o layers/{}.tif

Other changes

• (ProgressIndicator) Fix bugs found when writing image sequences.
• (apps) Remove vc_layers since it's no longer needed.
• (apps) vc_layers_from_ppm will automatically create the output directory if it doesn't exist, but the parent must exist.
• (apps) Improve saving logic in vc_render to support auto-named image sequences. Also adds --compression flag.
• (ImageIO) Add WriteImageSequence. Also adds padding to WriteImageOptions for controlling sequence padding.
• (util/String) Add partition function for splitting a string on a separator, similar to the Python function of the same name.
• (graph) Add nodes to support all of the above.