LOK: the Math in-place helper erased the painted tile

PaintAllInPlaceOnTile resized the tile device with the default erase,
so every Impress tile painted during text edit came back blank (black on
macOS). Keep the content, as the chart helper does.

diff --git a/sfx2/source/view/lokstarmathhelper.cxx b/sfx2/source/view/lokstarmathhelper.cxx
index 78f27b9..ff2becb 100644
--- a/sfx2/source/view/lokstarmathhelper.cxx
+++ b/sfx2/source/view/lokstarmathhelper.cxx
@@ -220,7 +220,7 @@ void LokStarMathHelper::PaintAllInPlaceOnTile(VirtualDevice& rDevice, int nOutpu
     const int nPartForCurView = pCurView->getPart();
 
     // Resizes the virtual device to contain the entries context
-    rDevice.SetOutputSizePixel({ nOutputWidth, nOutputHeight });
+    rDevice.SetOutputSizePixel({ nOutputWidth, nOutputHeight }, /*bErase*/false);
 
     auto popIt = rDevice.ScopedPush(vcl::PushFlags::MAPMODE);
     MapMode aMapMode(rDevice.GetMapMode());