Discovered and fixed some of the most obvious and major/minor memory allocation goofs...

Discovered and fixed some of the most obvious and major/minor memory allocation goofs in the scene graph rendering code.

The next major bottleneck in the scene graph with regard to huge memory allocation is the problem that all events
are currently propagated recursively throughout the whole scene graph by G2DParentNode.
J-P and I considered adding a separate listener registration mechanism as in DOM. While at it, we must specify better
how the scene graph event handling is performed and write it down.

git-svn-id: https://www.simantics.org/svn/simantics/sysdyn/trunk@18922 ac1ea38d-2e2b-0410-8846-a27921b304fc