With double-buffering enabled and typing quickly, one saw blinks in the
text sometimes. The reason was that PaintDesktop() painted directly
outside SwViewShell::Paint().

The problem is more visible with VCL_DOUBLEBUFFERING_AVOID_PAINT=1,
loading a simple document + pressing a key draw an unexpected white
rectangle.

Given that InvalidateWindows() already calls PaintDesktop() inside
Paint() implicitly (via SwViewShell::Paint()), fix the problem by just
avoiding the DLPrePaint2() + PaintDesktop() + DLPostPaint2() calls.

Change-Id: Ib8ce26a0bdd526bf85d362f0e865dd61d29f6d11