Add bigger terrain tile brushes to the map editor.

[SoScared]

A simple change to help with a long-standing annoyance with the editor. E.g. placing water tiles or making interior maps you're currently stuck with 1x1 and 2x2 tiles which on big maps is a pain to work with. Until a more fancy system can get in place there should definitely be something like 2x2, 4x4 and 6x6 tiles in the editor palette for certain tiles. There's more than enough room in the terrain - tiles tab. The interior tabs have their own group system.

relevant tiles: water, snow, grass, desert, interior floor (dark-268, light-275), and wall randomized 340 to 349.

Related OpenRA issue ticket: OpenRA/OpenRA#10752