csg: Don't use a namespace in a .h

Not every file including a .h may want to have std namespace imported.
"using namespace" should be restricted to cpp files.

Signed-off-by: Celelibi <celelibi@gmail.com>
CsgTree: Prefix members with m_

Because it's a convention.

Signed-off-by: Celelibi <celelibi@gmail.com>
BoundingBox: Reindentation

Because of reasons.

Signed-off-by: Celelibi <celelibi@gmail.com>
csg: Compute the bounding box of every node

Computation is not clever, it just transform the 4 corners of the
natural bounding box and create the new bounding box of that tranformed
bounding box.

Signed-off-by: Celelibi <celelibi@gmail.com>
csg: Make most methods virtual

They should be virtual in msot case.

Signed-off-by: Celelibi <celelibi@gmail.com>