Revert "CppunitTest_tiledrendering: disable SwVirtFlyDrawObj assert for now"

This is a real problem, which is fixed in a following commit -- tiled
rendering does not currently set the viewport at all, which can result
in an empty (or otherwise incorrect viewport), so this assert is definitely
valid.

This reverts commit fb0b42b89af95b42cc6caadf8c22321e5c9386e8.
Revert "Tiled Rendering: ensure rendered area is visible."

This reverts commit ff6a38b76df80d0f907f2f92cb851dea43c91d81.
Tiled Rendering: ensure rendered area is visible correctly.

Using SetVisArea can cause problems due to various internal
state getting confused, however this gives correct output
without viewport assertion problems or missing content.

Change-Id: I8d206ab201167ea96a0ac853187861e8c3d541e5
libreofficekit_tilederendering: test all 4 tiles.

The use of VisPortChgd ensures that the tiles all render as
expected, i.e. that the pixels match 1:1 irrespective of actual tile
size (for identical zoom factors and document areas).

Change-Id: Ib1e1df4f8257546c2f7993a8160c309a52037d8b
We're using the same rectangle for both calls, let's reuse it.

Change-Id: I884832fc92e0fe9c10711d9fdaf103076a05f721