Improve scroll performance by using the tile manager less

This pull request is a workaround to improve the scrolling performance of okular for typical use cases on 4k screens.

The workaround fixes the following issue: Open a pdf that has multiple letter-size pages. Make the window size at least about 4000x2000 pixels (eg maximize Okular on a 4k screen). Set the memory usage to "Normal" or higher in the "Performance Tuning" preferences. Set the "View Mode" to "Single Page" and "Fit Width". Now scroll the pdf up and down.

The issue is that Okular seems to have to rerender parts of the pdf when scrolling. Since the memory usage is set to normal, Okular should not need to rerender since it should have a cache of rendered pages. The issue is particularly noticeable for pdfs that take a long time to render.

The cause of the issue seems to be related to the TilesManager. If TilesManager is not used, then the issue does not occur. This pull request makes TilesManager not used for the above use case on a 4k screen.