[LyX/master] Fix instant preview for documents not specifying a specific output format.

[Enrico Forestieri]

commit b82e0d6d7f30dc327bba0f4364a5e29cc440e09b
Author: Enrico Forestieri <for...@lyx.org>
Date:   Mon Apr 27 12:27:42 2015 +0200

    Fix instant preview for documents not specifying a specific output format.
    
    Fixes the regression introduced at 0bb378ba.

diff --git a/src/graphics/PreviewLoader.cpp b/src/graphics/PreviewLoader.cpp
index 3fd8fef..6dfec9b 100644
--- a/src/graphics/PreviewLoader.cpp
+++ b/src/graphics/PreviewLoader.cpp
@@ -587,7 +587,13 @@ void PreviewLoader::Impl::startLoading(bool wait)
 
        LYXERR(Debug::LATEX, "Format = " << 
buffer_.params().getDefaultOutputFormat());
        string latexparam = "";
-       OutputParams::FLAVOR flavor = buffer_.params().getOutputFlavor();
+       bool docformat = !buffer_.params().default_output_format.empty()
+                       && buffer_.params().default_output_format != "default";
+       // Use LATEX flavor if the document does not specify a specific
+       // output format (see bug 9371).
+       OutputParams::FLAVOR flavor = docformat
+                                       ? buffer_.params().getOutputFlavor()
+                                       : OutputParams::LATEX;
        if (buffer_.params().encoding().package() == Encoding::japanese) {
                latexparam = " --latex=platex";
                flavor = OutputParams::LATEX;