add map vantage points

The vantage point is supposed to be a distinctive view on a map - like the
angle from which the preview picture was taken.

This commit adds three commands: clearvantagepoint, setvantagepoint and
gotovantagepoint. None has any parameters, all are only allowed in edit
mode. A map can only have one vantage point.
Do "SCROLL to change player" visible in any standard aspect ratio

Above text was cut for aspect ratio 5:4 (quite much) and 4:3 (a little) for
previous value "left = (VIRTW*3/2)*6/8"