Fixes https://github.com/derf/feh/issues/580

Note that Imlib2 does not support HEIC/HEIF images out of the box. However, the
imlib2-heic loader (https://github.com/vi/imlib2-heic) does.

Closes #579

This fixes two memory bugs that only manifest with exif=1 and long-running
slideshows.

* when feh loads an image, it writes exif data to file->ed. Previously, this
  data was never free'd, causing a memory leak on subsequent loads of the same
  file.
* As file->ed is never free'd, the accumulated EXIF data consumes a significant
  amount of memory over time. with slideshow-delay = 10 and two days of
  runtime, feh may exceed 1 GB of memory usage. If the slideshow is so large
  that feh does not encounter the same image twice in this time, this is not
  detected as a memory leak, as each EXIF data chunk is referenced from the
  filelist.

See <https://github.com/derf/feh/issues/553> for details.

Closes #553

This works around a regression in Imlib2, which makes (un)loadable file
detection quite slow when handling e.g. large video files. See
<https://phab.enlightenment.org/T8739> and
<https://github.com/derf/feh/issues/505> for details.

Closes #505

Closes #532

Closes #521

feh_file_info_load is called even if file_info is already populated, so
the original file_info struct is never freed. This results in a leak of
~44 Byte for each subsequenc image load

Building feh 3.3 on CentOS 7 x86_64 warns `curl_quit_function` in `imlib.c` is unused:

```
cc -g -O2 -Wall -Wextra -pedantic -std=c11 -D_POSIX_C_SOURCE=200809L -D_XOPEN_SOURCE=700 -DHAVE_LIBCURL -DHAVE_VERSCMP -DHAVE_LIBXINERAMA -DHAVE_LIBEXIF -DPREFIX=\"/usr/local\" -DPACKAGE=\"feh\" -DVERSION=\"3.3\"   -c -o imlib.o imlib.c
imlib.c:545:12: warning: ‘curl_quit_function’ defined but not used [-Wunused-function]
 static int curl_quit_function(void *clientp,  curl_off_t dltotal,  curl_off_t dlnow, curl_off_t ultotal, curl_off_t ulnow)
            ^
```
The `curl_quit_function` code was added in response to pull [#435](https://github.com/derf/feh/pull/435)

In issue [#485](https://github.com/derf/feh/issues/485) a fellow CentOS 7 user had an error building feh because CentOS 7 is locked into an old version of libcurl. In the fix, a version guard was wrapped around the `curl_easy_setopt` call, but the rest of the code was unchanged.

Since I don't want to maintain a local build of libcurl, I looked at the curl docs and noticed there is an older callback which serves the same purpose: https://curl.haxx.se/libcurl/c/CURLOPT_PROGRESSFUNCTION.html

The difference between `PROGRESS` and `XFERINFO` is the callback's argument types, with `PROGRESS` using `double` and `XFERINFO` using `curl_off_t`: https://curl.haxx.se/libcurl/c/CURLOPT_XFERINFOFUNCTION.html

The callback's return value logic and use of `CURLOPT_NOPROGRESS` is the same.

For context, the latest libcurl RPM I'm getting from yum updates is `libcurl-7.29.0-54.el7_7.2.x86_64`. The "stable" versions of other distros may encounter similar issues. The CentOS 7 "End of Life" date is 2024-06-30 so you should hear the end of this by then, at least from us pesky CentOS users.

Closes #485

sig_exit may be changed by a signal handler, so its value should always be
read from RAM.

See also #435

Fixes non-random behaviour when randomizing file lists several times per
second.

Closes #349

Uses the camera-generated thumbnail to display RAW images that could otherwise not be loaded.

This introduces a new feh_should_ignore_image function which is called
at appropriate places in those modes to skip images which are loadable but
undesired.

* Prevents nasty loading loops

* Prevents zombie subprocesses

* Fixes the conversion timeout detection routine

This option allows you to change the default imlib2 image cache size of 4 MiB.

Similar to the `curl` command-line tool.

The check if buffer == NULL is always false, since buffer is an
autoamtic variable allocated when entering the function.  What we
instead want to do is to check if the string is empty after the call to
exif_get_info(), since that means we could not read any exif
information.
When the code once more is enabled, I discovered that we need to copy
the information string into info_buf as well as into  buffer, since it
is the former that is used to print the exif information on top of the
picture.  Without this change, imlib warns about trying to write NULL
strings.