CsgOperation: recursively delete on destruction

When a CsgOperation is deleted, also delete the children nodes. If this
behavior is not wanted, set the children to NULL before.

Signed-off-by: Celelibi <celelibi@gmail.com>
CsgTree: Add a method to empty a CsgTree

This method will be used to clear the CSG tree on demand.

Signed-off-by: Celelibi <celelibi@gmail.com>
InterfaceCSG: Clear the CSG on demand

Signed-off-by: Celelibi <celelibi@gmail.com>
MainWindow RenderImg: Comment fixing

Signed-off-by: Celelibi <celelibi@gmail.com>
renderImg: Load and display a pgm image

The displayed image is buggy, but it display something!

Signed-off-by: Celelibi <celelibi@gmail.com>
RenderImg: Load any size of image

Because fuck OpenGL, I can't load a texture whose width isn't a multiple
of 4.

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