WINTERMUTE: Fix computation of _renderRect.

This reverts the changes in 4cf1d671.
Fixes the "notebook bug" - i.e. the notebook being only partially
drawn - in Dirty Split.
WINTERMUTE: Fix offset calculation with partial rects when sprites are mirrored.

This fixes the dress bug, aka Rosemary appearing "jigsawed" when passing
over her with the cursor when she is facing east.
See bug #3592907