Fix zoom ratio calculation

[ulteq]

This simplifies the logic behind the automatic zoom ratio calculation, which is used by both --auto-zoom and --scale-down.

When merged this will:

• Improve the performance (especially in fullscreen mode)
• Fix the w command when both --scale-down and --keep-zoom-vp are enabled
• Fix --auto-zoom not being triggered on window resize events when --scale-down is enabled
• Fix --auto-zoom not being applied to the first image
• Fix --auto-zoom conflicting with manual zoom
• Fix --zoom <percent> --auto-zoom logic according to the man feh description
• Fix feh_draw_checks not taking the zoom level into account properly
• Fix feh_draw_checks being called before the zoom level is adjusted
• Prevent --zoom <percent> from blocking --scale-down in fullscreen / fixed geometry mode
• Prevent --keep-zoom-vp from blocking the dynamic window resizing mechanism
• Prevent automatic recalculation of the zoom ratio when --keep_zoom_vp is enabled
• Prevent winwid-> im_x and winwid-> im_y from being set to invalid values
  (when switching between images of different sizes)
• Simplify the way --keep-zoom-vp is handled internally

Fixes: #229
Fixes: #244
Fixes: #307

Closes: #358
Closes: #361
Closes: #362

[derf]

Hmm. I'm happy that someone is attempting a cleanup of this mess, though this still needs some testing (and, possibly, polishing).

Right now the first slide of a feh --fullscreen slideshow is not centered properly. I'll let you know if I find anything else (or come up with a patch for it, in which case I'll happily merge the request unless I find other issues).

Expect delays though, I don't have much time at the moment.

[ulteq]

though this still needs some testing

I absolutely agree.

Expect delays though

No problem. We don't have to rush this.

[ulteq]

Right now the first slide of a feh --fullscreen slideshow is not centered properly.

int had_resize = winwid->had_resize || resize; solved it.

[ulteq]

@derf This is ready for a second review.

[derf]

hmm, this still causes some behaviour changes in feh. fullscreen mode no longer zooms out images which are too large, and due to the re-use of the geometry flags settings leaving fullscreen mode will cause non-tiling feh windows to be as big as the screen (instead of fitted to the size of the image, which is usually smaller). Also, in non-tiling mode, feh will no longer resize its window when changing images.

I've yet to decide which of these I consider to be an issue and which I don't (and do more testing), so this is just the current status to make sure i don't forget it.

[ulteq]

fullscreen mode no longer zooms out images which are too large

I modified this condition in order to replicate the old behavior: 9d959f0#diff-24777376c7590882b07457017fd284ceR459

Also, in non-tiling mode, feh will no longer resize its window when changing images.
leaving fullscreen mode will cause non-tiling feh windows to be as big as the screen

Both of this happened because I enabled fixed geometry mode by default in order to avoid the flickering in tiling windows: 203e8ff#diff-24777376c7590882b07457017fd284ceL326

I have now replaced this code change with a slightly modified version of the original code: 66a1eb8

Flickering in tiling window mode can now be circumvented by passing an empty string to the --geometry option. This makes feh launch with fixed geometry mode enabled. We could also add a new --fixed-geometry option instead.

[ulteq]

Please let me know if there is anything I can do to help you make this pull request ready for merge.

[derf]

Actually, just poking me again was sufficient. I couldn't find any issues with this version :)

Thanks a lot for the patches and sorry for the long wait!