windows opengl: fix CppunitTest_writerperfect_writer

(cherry picked from commit b5df051c)

Change-Id: I4a7199633d0d940ae009ac144989da7d6c9262ec
Reviewed-on: https://gerrit.libreoffice.org/18399Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Andras Timar <andras.timar@collabora.com>

vcl/win/source/gdi/winlayout.cxx

@@ -1763,6 +1763,10 @@ bool UniscribeLayout::DrawCachedGlyphs(SalGraphics& rGraphics) const
 
         for (int i = nMinGlyphPos; i < nEndGlyphPos; i++)
         {
+            // Ignore dropped glyphs.
+            if (mpOutGlyphs[i] == DROPPED_OUTGLYPH)
+                continue;
+
             assert(mrWinFontEntry.GlyphIsCached(mpOutGlyphs[i]));
 
             const OpenGLGlyphCacheChunk& rChunk = mrWinFontEntry.GetCachedGlyphChunkFor(mpOutGlyphs[i]);