feh_draw_info used to iterate line-wise over the info output, but now we need
to cache it somewhere to get the length of the longest line.

The info text used to be located 2px away from the bottom / left edges.  Now
that --draw-tinted has been implemented, that leaves us with an ugly edge, so
move it directly to the edge.

By default, feh draws white text with a black drop-shadow, which is well
readable in most, but not all cases.  The --draw-tinted option draws a
semi-transparent black box around all texts.  This improves readability, but
does not look as nice, so it won't become default behaviour.

It was suggested to use #7f7f7f7f for the background, but I decided that
for small images surrounded by a black border.

feh_draw_info used to unconditionally swallow the last character of a line,
assuming that it must be a newline. However, the very last line of a program's
output may not end in a newline, in which case its last output character was
omitted.

This prevents a (highly unlikely) case of an attacker knowing feh's PID and
the user's URL rewriting user files by means of a TOCTTOU attack.

It is still possible to _create_ arbitrary files via dangling symlinks. That
will be fixed once I switch from wget to libcurl.

Signed-off-by: Daniel Friesel <derf@finalrewind.org>