making a disabled image from a 1 bit depth icon gives index asserts

cause the disabled color cannot be mapped to the 2 entries in the
palette, so use 8bit dests if the src is 1bit

regression from

commit 507d0ded
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Tue Dec 1 12:18:57 2015 +0000

    keep disable image at same depth as original

seen when clicking on the image loaded from tdf#100632

Change-Id: Ic69bccfa0fc86707b4fd9e757d8374d80e4b0071

vcl/source/bitmap/BitmapProcessor.cxx

@@ -59,7 +59,14 @@ BitmapEx BitmapProcessor::createDisabledImage(const BitmapEx& rBitmapEx)
 {
     const Size aSize(rBitmapEx.GetSizePixel());
 
-    Bitmap aGrey(aSize, rBitmapEx.GetBitCount());
+    //keep disable image at same depth as original where possible, otherwise
+    //use 8 bit
+    sal_uInt16 nBitCount = rBitmapEx.GetBitCount();
+    if (nBitCount < 8)
+        nBitCount = 8;
+    const BitmapPalette* pPal = nBitCount == 8 ? &Bitmap::GetGreyPalette(256) : nullptr;
+    Bitmap aGrey(aSize, nBitCount, pPal);
+
     AlphaMask aGreyAlpha(aSize);
 
     Bitmap aBitmap(rBitmapEx.GetBitmap());