[LyX/master] Try hard to obtain an instant preview.

[Enrico Forestieri]

commit 691fdea346eb5643bff3f84017629d966a90af05
Author: Enrico Forestieri <for...@lyx.org>
Date:   Mon Apr 27 17:22:25 2015 +0200

    Try hard to obtain an instant preview.
    
    When the dvipng route is selected and latex fails, the lyxpreview2bitmap.py
    script simply bails out. The failure may be due to using a specific pdf
    only feature in ERT (for example, the inclusion of a pdf image).
    In this case the procedure may succeed simply trying to use pdflatex
    and the legacy conversion method.

diff --git a/lib/scripts/lyxpreview2bitmap.py b/lib/scripts/lyxpreview2bitmap.py
index 5655bf3..b387529 100755
--- a/lib/scripts/lyxpreview2bitmap.py
+++ b/lib/scripts/lyxpreview2bitmap.py
@@ -413,7 +413,9 @@ def main(argv):
     # Compile the latex file.
     latex_status, latex_stdout = run_latex(latex, latex_file, bibtex)
     if latex_status:
-      return (latex_status, [])
+        progress("Using the legacy conversion method and pdflatex (latex 
failed)")
+        return legacy_conversion_step1(latex_file, dpi, output_format, 
fg_color,
+            bg_color, "pdflatex", True)
 
     # The dvi output file name
     dvi_file = latex_file_re.sub(".dvi", latex_file)