Related to issue #303

Before that zoomdiff was a merely heuristic.

Related to issue #303

Fixes issue #302

When sxiv is built with AUTORELOAD=nop if something is feed via stdin it
will start to infinitely call select() leading to a 100% CPU usage.

Thanks to @sdx23 for helping in analyzing that problem!

Fixes issue #297.

Non-fitting text is truncated by simply cutting it off.

win_textwidth() has been replaced by a simple macro wrapper around
win_draw_text() with a maximum width of zero, which results in a dry-run that
still calculates the number of pixels used by the text, much like snprintf(3)
with a buffer length of zero.

Code under a different license should be kept in a separate file. This
implemention is a single header file with ~65 lines, so it better fits this
requirement.

Fixes #276

Instead of rendering the entire filename at once, Xft will let us do it
character by character. This will allow sxiv to query fontconfig for
a font that can provide any missing codepoints, if needed.

A known issue of this patch is that the "..." dots rendering will not
work properly for very long multibyte filenames. That is because we
cannot easily predict the final width of the rendered filename before
drawing it. I couldn't figure out a clean way to deal with this, so I
ended up just truncating the offending filenames.

Fixes issue #295

The imlib2 TGA loader returns an imlib image handle without any actual data
when given a text file like this:

    T
    Content-Type: application/javascript
    Content-Length: 3836
    Last-Modified: Wed, 23 Sep 2015 12:25:47 GMT
    Etag: "56029a4b-efc"
    Expires: Sat, 20 Aug 2016 15:14:33 GMT
    Cache-Control: max-age=604800, public
    Accept-Ranges: bytes

Fortunately, `imlib_image_get_data()` returns NULL in this case, so that we can
use it as an additional check when opening files.

Fixes issue #293.

Our own default values go into accompanying macros prefixed with DEF_.

OpenBSD make supports VPATH but not pattern rules. Everything in the new
Makefile should work no matter what make program is used.

The new Makefile should work with the make programs shipped with FreeBSD,
NetBSD and OpenBSD.

The config.mk file is now optional and only needs to be created if one wants to
persistently overwrite default macro values.

Features used in the Makefile that are not yet in the POSIX standard:

  - Advanced macro assignment operators '+=' and '?=' [1]
  - Special target .PHONY [2]
  - Pattern rules [3]; only needed when $srcdir != '.'. For every pattern rule
    there is an inference rule with the same effect. Hopefully, the inference
    rules get picked up by make programs not supporting pattern rules.
  - Silently including multiple possibly not exisiting files [4] [5]

[1] http://austingroupbugs.net/view.php?id=330
[2] http://austingroupbugs.net/view.php?id=523
[3] http://austingroupbugs.net/view.php?id=513
[4] http://austingroupbugs.net/view.php?id=333
[5] http://austingroupbugs.net/view.php?id=518

None of the mouse mappings uses a keyboard modifier, making it possible to
access the most basic features by only using the mouse.

Next/previous image with left button depending on cursor position, middle
button for dragging, right button for switching to thumnail mode and wheel for
zooming.

Users can keep the old behaviour by simply not adapting the changes to the
buttons array in config.def.h to their config.h file.

No need to give cursor name. reset_cursor() is now the only place to decide
what cursor to fall back to.

This makes mouse panning more direct and faster.

Closes #285.

Verbose when build with `V=1'.

Fixes issue #167. Long after closing this issue (kind of wontfix) we have switched to
GNU make and can thus use its features like VPATH.