all games: Add brush model support for glow outlines FX

[copperpixel]

Description

This PR adds support for rendering glow outlines on brush models. It achieves this by installing a custom brush renderer during glow object's DrawModel() routine, allowing the model's mesh to be drawn with the glow texture. You can then set a tf_glow entity's target to a brush entity to make it glow.

^ demonstration with lambda brushwork

A hack is currently used where a studio model is drawn with a scale of 0 to initialize the correct render states before drawing a brush model's glow outline, allowing shader stencils to apply properly. Proper fix most likely would require modifications of engine code.

Closes:

• https://github.com/ValveSoftware/Source-1-Games/issues/6381