Wip/spacer gizmo selection

[Raven358]

Description:

Initial WIP implementation for the Spacer editor.

Includes:

• object selection using actual mesh geometry rather than only bounding boxes;
• Im3d transform gizmo;
• selection outline;
• required editor shaders and integration.

This is an experimental implementation and still needs cleanup/review. A significant part of the initial code was AI-assisted/generated.

Known issues:

• gizmo manipulators can stretch when close to the screen edge;
• selection outline may create visual noise on textures with an alpha channel when viewed very closely.

[Try]

Hi, @Raven358 and thanks for PR!

As we discussed on discord, I'm not going to take patch as-is, but rather use it as a base to rewrite optimal implementation.
Here current state of selection (also property view is updates accordingly)