[LyX/2.2.x] Backport fix for #9778.

[Richard Heck]

commit 11d681633053090f1a499a900d76dd7117937e55
Author: Enrico Forestieri <for...@lyx.org>
Date:   Sun Oct 15 12:12:34 2017 -0400

    Backport fix for #9778.
    
    From Enrico.
---
 src/frontends/qt4/GuiApplication.cpp |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/src/frontends/qt4/GuiApplication.cpp 
b/src/frontends/qt4/GuiApplication.cpp
index 5806558..36d6926 100644
--- a/src/frontends/qt4/GuiApplication.cpp
+++ b/src/frontends/qt4/GuiApplication.cpp
@@ -35,6 +35,7 @@
 #include "BufferView.h"
 #include "CmdDef.h"
 #include "Color.h"
+#include "Converter.h"
 #include "CutAndPaste.h"
 #include "ErrorList.h"
 #include "Font.h"
@@ -241,6 +242,17 @@ vector<string> loadableImageFormats()
                // special case
                if (ext == "jpeg")
                        ext = "jpg";
+               else if (lyxrc.use_converter_cache
+                        && (ext == "svg" || ext == "svgz")
+                        && theConverters().isReachable("svg", "png"))
+                       // Qt only supports SVG 1.2 tiny. See #9778. We prefer
+                       // displaying the SVG as in the output. However we
+                       // require that the converter cache is enabled since
+                       // this is expensive. We also require that an explicit
+                       // svg->png converter is defined, since the default
+                       // converter could produce bad quality as well.
+                       // This assumes that png can always be loaded.
+                       continue;
                fmts.push_back(ext);
        }