Building/Units on-screen visibility

[Balthazar]

Zuzuf, i`ve did a few new tests and can point one usefull thing.

When moving the screen area, all the buildings and units begin to draw AFTER their center gets in the screen area. That`s not right. The buildings should appear once any of it`s corners are gets into screen area. This can be asilly noticed if the building have big size. The bigger - the worser it looks.

Should be easy to fix.

[zuzuf]

good idea, the visibility detector should check the selection rectangle.