FACE-TO-BRUSH BIG ISSUE

[karakakarkarkar]

Need skip CONTENTS_SKY and CONTENT_SOLID (need check contents from leafs with this faces, then check 'if leaf content is sky/solid then this is trash faces) faces to make exported .map cleaned from trash.

@SamVanheer Real skybox has contents_empty leaf (visibled to players). And leaf 0 faces (or contents solid) can be ignored from .map export because is not visibled and planeside outside map.