Fix alt text when multiple instances of a png figure are added in different chunks

[cgrandin]

If include_graphics() is used in two or more different chunks to load the same image file (same exact filename) then the alt text will cause malformed tag structure and the alt text will not appear on the PDF because the routine assumes exactly one instance of a filename with one instance of an alt text phrase. Need to modify post_process_add_alt_text() so that a filename can be used multiple times and be matched with the correct alt text phrases.

[andrew-edwards]

Or just make a copy of the .png with a slightly different filename and load each file separately in each chunk.
Likely to not be a common situation?