vcl: Use offscreen rendering for native GTK widgets with OpenGL backend

Change-Id: I142e1b34e54a3cbe149e5af9adeba1250a4ca9a9

vcl/Library_vclplug_gtk.mk

@@ -65,6 +65,7 @@ $(eval $(call gb_Library_use_externals,vclplug_gtk,\
 	cairo \
 	dbus \
 	gio \
+	glew \
 	gtk \
 	gthread \
 	icuuc \

vcl/unx/gtk/gdi/salnativewidgets-gtk.cxx

@@ -41,6 +41,8 @@
 #include "fontmanager.hxx"
 #include <vcl/decoview.hxx>
 
+#include <vcl/opengl/OpenGLHelper.hxx>
+
 typedef struct _cairo_font_options cairo_font_options_t;
 const char* const tabPrelitDataName="libreoffice-tab-is-prelit";
 
@@ -533,6 +535,10 @@ void GtkData::initNWF( void )
     if( pEnv && *pEnv )
         GtkSalGraphics::bNeedPixmapPaint = true;
 
+    // use offscreen rendering when using OpenGL backend
+    if( OpenGLHelper::isVCLOpenGLEnabled() )
+        GtkSalGraphics::bNeedPixmapPaint = true;
+
     #if OSL_DEBUG_LEVEL > 1
     std::fprintf( stderr, "GtkPlugin: using %s NWF\n",
              GtkSalGraphics::bNeedPixmapPaint ? "offscreen" : "direct" );