vifm: Improve image preview with fallback

If a locally compiled version of viu exists it will use this for
full image preview display (full-color image). If it does not
exist it will instead fall back to the system viu and use
block-wise display for the preview. Location that local viu is
expected is `~/.local/bin/viu`.

The reason behind this is a bug (or at least unwanted
functionality) in viu which makes it not work correctly from
within vifm. You will have to fix this issue and compile a local
version of viu which vifm assumes to be in the local binary
directory and uses to display the pretty images.

terminal/.config/vifm/vifmrc

@@ -444,7 +444,11 @@ filextype *.bmp,*.jpg,*.jpeg,*.png,*.gif,*.xpm
         \ {View in gpicview}
         \ gpicview %c,
 fileviewer *.bmp,*.jpg,*.jpeg,*.png,*.gif,*.xpm
-        \ viu -b -w %pw -h %ph -s %c
+        " to circumvent the bug: https://github.com/atanunq/viuer/issues/44
+        " thus, change the viuer library and build it with the new lib locally
+        " put it into the local bin directory and you can see full pics. 
+        " Otherwise falls back to block-sized display
+        \ exist ~/.local/bin/viu && env -u COLORTERM ~/.local/bin/viu -w %pw -s %c %pd || env -u COLORTERM viu -b -w %pw -s %c %pd
 
 " OpenRaster
 filextype *.ora