tdf#115297: alternative fix for displaying 1bit images

Previous fixes for 1bit monochrome / paletted images produced some
regressions, so here is attempt to fix problem at the root.

Partially reverted 66dbd4da and
fixed in a different way without tdf#115297, tdf#114726 and related.

Change-Id: I6849ed5ac41770ba905c930065c80e58509dba2e
Reviewed-on: https://gerrit.libreoffice.org/50454Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

vcl/headless/svpgdi.cxx

@@ -1200,10 +1200,33 @@ void SvpSalGraphics::copyBits( const SalTwoRect& rTR,
         cairo_surface_destroy(pCopy);
 }
 
+namespace
+{
+    bool isBlackWhite(const SalBitmap& rBitmap)
+    {
+        const SvpSalBitmap& rSrcBmp = static_cast<const SvpSalBitmap&>(rBitmap);
+        const BitmapBuffer * pSourceBuffer = rSrcBmp.GetBuffer();
+        const BitmapPalette & rPalette = pSourceBuffer->maPalette;
+
+        return (
+            rPalette.GetEntryCount() < 2 ||
+
+            (rPalette.GetEntryCount() == 2 &&
+            rPalette[0] == Color(COL_BLACK) &&
+            rPalette[1] == Color(COL_WHITE) ) ||
+
+            (rPalette.GetEntryCount() == 2 &&
+            rPalette[1] == Color(COL_BLACK) &&
+            rPalette[0] == Color(COL_WHITE) )
+            );
+    }
+}
+
 void SvpSalGraphics::drawBitmap(const SalTwoRect& rTR, const SalBitmap& rSourceBitmap)
 {
-    if (rSourceBitmap.GetBitCount() == 1)
+    if (rSourceBitmap.GetBitCount() == 1 && isBlackWhite(rSourceBitmap))
     {
+        // This way we draw only monochrome b/w bitmaps
         MaskHelper aMask(rSourceBitmap);
         cairo_surface_t* source = aMask.getMask();
         copySource(rTR, source);

vcl/source/gdi/pngread.cxx

@@ -535,19 +535,11 @@ bool PNGReaderImpl::ImplReadHeader( const Size& rPreviewSizeHint )
         {
             switch ( mnPngDepth )
             {
-                case 1 :
-#if defined(UNX) && !defined(MACOSX)
-                    // 1bpp indexed images are so badly mishandled by rest of LO on X11 that we
-                    // don't even bother, and turn them into 8bpp indexed ones with just two palette
-                    // entries instead.
-                    mnTargetDepth = 8;  // we have to expand the bitmap
-#endif
-                    mbPalette = false;
-                    break;
                 case 2 :
                     mnTargetDepth = 4;  // we have to expand the bitmap
                     mbPalette = false;
                     break;
+                case 1 :
                 case 4 :
                 case 8 :
                     mbPalette = false;