process only half width & height in closeup mode

No need to calculate the whole 100% roi if are viewing at 200%. This
allows the viewing area in closeup mode to go all the way to outer
edges of the image without trying to calculate beyond the image edge.
fix zoom jumping at edges

This reverses a change from
darktable-org/darktable@54f0f75c23f80b037715c4a47be58c9118698a1f. The
zoom jumping that commit fixed is now solved by the prior commit,
which only calculates the viewable rectangle in closeup mode.