Incorrect positioning of destroy/build grid

[Cherry Soup]

If start to build road/aqueduct/homes or enter destroy mode, press mouse, and then move camera, grid is moving with the camera, and not snaps to starting position.
In original Caesar III it's snaps normally.

[dalerank]

fix #304. Improved rendering selected area build/destroy level

→ <<cset 825690a54e98>>