This incorrectly reversed the priority ordering of +latex-viewers. This
is an artifact from a rejected approach at short-circuiting
+latex-viewer, which was replaced with cl-block->cl-return.
Reported by @UndeadKernel
The +zathura, +skim, +okular and +pdf-tools module flags have been
removed in favor of +latex-viewers, which takes a list of symbols. Its
order determines the priority. The first viewer found on your system is
used.
If none of these viewers are found, it will fall back to
latex-preview-pane.
