rendercontext: Make the Gtk+ native widgets work with double-buffering.

Change-Id: Ic7101ab7cbaa7fd5f6b073a73ef0c0088b1af4c6
Reviewed-on: https://gerrit.libreoffice.org/15980Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>

include/vcl/salnativewidgets.hxx

@@ -246,11 +246,12 @@ enum class ControlState {
     HIDDEN       = 0x0010,
     DEFAULT      = 0x0020,
     SELECTED     = 0x0040,
-    CACHING_ALLOWED  = 0x8000,  // set when the control is completely visible (i.e. not clipped)
+    DOUBLEBUFFERING = 0x4000,  ///< Set when the control is painted using double-buffering via VirtualDevice.
+    CACHING_ALLOWED = 0x8000,  ///< Set when the control is completely visible (i.e. not clipped).
 };
 namespace o3tl
 {
-    template<> struct typed_flags<ControlState> : is_typed_flags<ControlState, 0x8007f> {};
+    template<> struct typed_flags<ControlState> : is_typed_flags<ControlState, 0xc007f> {};
 }
 
 /* ButtonValue:

vcl/source/outdev/nativecontrols.cxx

@@ -18,6 +18,7 @@
  */
 
 #include <vcl/outdev.hxx>
+#include <vcl/virdev.hxx>
 #include <vcl/window.hxx>
 
 #include <vcl/salnativewidgets.hxx>
@@ -300,6 +301,9 @@ bool OutputDevice::DrawNativeControl( ControlType nType,
     if (aTestRegion == vcl::Region(rControlRegion))
         nState |= ControlState::CACHING_ALLOWED;   // control is not clipped, caching allowed
 
+    if (dynamic_cast<VirtualDevice*>(this))
+        nState |= ControlState::DOUBLEBUFFERING;
+
     bool bRet = mpGraphics->DrawNativeControl(nType, nPart, screenRegion, nState, *aScreenCtrlValue, aCaption, this );
 
     return bRet;

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

@@ -860,12 +860,9 @@ bool GtkSalGraphics::hitTestNativeControl( ControlType        nType,
  *  aValue:          An optional value (tristate/numerical/string)
  *  rCaption:      A caption or title string (like button text etc)
  */
-bool GtkSalGraphics::drawNativeControl(    ControlType nType,
-                            ControlPart nPart,
-                            const Rectangle& rControlRegion,
-                            ControlState nState,
-                            const ImplControlValue& aValue,
-                            const OUString& rCaption )
+bool GtkSalGraphics::drawNativeControl(ControlType nType, ControlPart nPart,
+        const Rectangle& rControlRegion, ControlState nState,
+        const ImplControlValue& aValue, const OUString& rCaption)
 {
     // get a GC with current clipping region set
     GetFontGC();
@@ -891,7 +888,7 @@ bool GtkSalGraphics::drawNativeControl(    ControlType nType,
     std::unique_ptr<GdkX11Pixmap> xPixmap;
     std::unique_ptr<GdkX11Pixmap> xMask;
     Rectangle aPixmapRect;
-    if( ( bNeedPixmapPaint )
+    if ((bNeedPixmapPaint || (nState & ControlState::DOUBLEBUFFERING))
         && nType != CTRL_SCROLLBAR
         && nType != CTRL_SPINBOX
         && nType != CTRL_TAB_ITEM