Add png dumping to LOK tiled rendering test.

This allows for easier visual comparisons (i.e. currently the test
would be failing for some tiles).

Change-Id: I5b174375b57ffe0edd2700fdec411a83669e4a34
LOK: use solar mutex for setPart.

Needed e.g. for changing slides in impress.

Change-Id: I2f5de40d4efbacde910e27225768979a98ff1c0a
Impress: implement tiled rendering.

However we cannot as of yet select between rendering just the slide,
just the notes, or both combined -- this simply defaults to whatever
mode the document was last opened in for now.

Change-Id: Ia8ec0280aab75a36e430aa04c47cee4fea2db974