tdf#88831 fix inverted textures when OpenGL is enabled

GLX returns a wrong value if the y coords are inverted. Most other
programs don't even ask for this (gnome-shell for example) and just
assumes "true" (and this works because most relevant X servers work
like this). We make this more robust and assume true only if the
returned value is GLX_DONT_CARE (-1).

Change-Id: I4800b3364fd00f5f4a8f5a459472bfa8d97827ba
Reviewed-on: https://gerrit.libreoffice.org/16860Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/opengl/OpenGLHelper.cxx

@@ -539,7 +539,11 @@ GLXFBConfig OpenGLHelper::GetPixmapFBConfig( Display* pDisplay, bool& bInverted
         }
 
         glXGetFBConfigAttrib( pDisplay, aFbConfigs[i], GLX_Y_INVERTED_EXT, &nValue );
-        bInverted = nValue == True;
+
+        // Looks like that X sends GLX_DONT_CARE but this usually means "true" for most
+        // of the X implementations. Investigation on internet pointed that this could be
+        // safely "true" all the time (for example gnome-shell always assumes "true").
+        bInverted = nValue == True || nValue == int(GLX_DONT_CARE);
 
         break;
     }