set a default delay if delay is 0 (#445)

gif spec [0] doesn't mention what to do when "Delay Time" is 0.

apng spec [1] states:

| If the the value of the numerator is 0 the decoder should render the
| next frame as quickly as possible, though viewers may impose a
| reasonable lower bound.

webp spec [2]:

| the interpretation of frame duration of 0 (and often <= 10) is
| implementation defined.

so it seems that it's safe to set a default delay for 0 delay frames,
which is what the older gif and webp loaders were already doing. do the
same for the imlib2 multi-frame loader as well.

[0]: https://www.w3.org/Graphics/GIF/spec-gif89a.txt
[1]: https://wiki.mozilla.org/APNG_Specification
[2]: https://developers.google.com/speed/webp/docs/riff_container#animation

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/445
Reviewed-by: eylles <eylles@noreply.codeberg.org>

image.c

@@ -54,6 +54,10 @@ enum { DEF_GIF_DELAY = 75 };
 enum { DEF_WEBP_DELAY = 75 };
 #endif
 
+#if HAVE_IMLIB2_MULTI_FRAME
+enum { DEF_ANIM_DELAY = 75 };
+#endif
+
 #define ZOOM_MIN (zoom_levels[0] / 100)
 #define ZOOM_MAX (zoom_levels[ARRLEN(zoom_levels) - 1] / 100)
 
@@ -539,7 +543,7 @@ static bool img_load_multiframe(img_t *img, const fileinfo_t *file)
 		imlib_context_set_blend(!!(finfo.frame_flags & IMLIB_FRAME_BLEND));
 		imlib_blend_image_onto_image(frame, has_alpha, 0, 0, sw, sh, sx, sy, sw, sh);
 		m->frames[m->cnt].im = canvas;
-		m->frames[m->cnt].delay = finfo.frame_delay;
+		m->frames[m->cnt].delay = finfo.frame_delay ? finfo.frame_delay : DEF_ANIM_DELAY;
 		m->length += m->frames[m->cnt].delay;
 		m->cnt++;
 		imlib_context_set_image(frame);