Revert "vcl: remove unnecessary resets"

This reverts commit 210f304108.

Reason for revert:
As Mike Kaganski explains in [1]:

> This is technically wrong. The bitmaps must not be used when they
> have active write access. Creation of the BitmapEx in theory may 
> scale the bitmaps (indeed, here they are created with the same size), > which would assert on active write access.
>
> Se tdf#156856


[1] https://gerrit.libreoffice.org/c/core/+/180142/comments/d55835ab_a4f52e04

Change-Id: I731b7ff2b17f33f886e7113ad48e30f87e9b80d6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/180211
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/source/bitmap/BlendFrameCache.cxx

@@ -127,6 +127,9 @@ BlendFrameCache::BlendFrameCache(Size const& rSize, sal_uInt8 nAlpha, Color cons
         }
     }
 
+    pContent.reset();
+    pAlpha.reset();
+
     m_aLastResult = BitmapEx(aContent, aAlpha);
 }