Fix non-native thumbnail height display
Fix row and sheet generation

My version of GM completely aborts if any source files are missing,
so we never generate any rows or sheets.

Now we check if the source exists.
If not, we create it.  If so, we use it.
Add a JS function to allow left/right arrow nav

Left goes back, right goes forward.

Up and down aren't mapped because people hit up and down
to scroll the page for large images or to read the rest
of the EXIF output.