Support LoadMasks for graphics quality adjustments

In Cyan's Ages, some objects (particularly wavesets) are set up with plLoadMask flags to ensure that they are only rendered on the highest quality graphics level and when pixel shaders are present. Conversely, Cyan often provides a "fallback" water surface that is just a plane with some animated textures.

Currently, korman never exports plLoadMasks on any objects, which results in wavesets always rendering at any graphics quality level (or in some cases not rendering if pixel shaders aren't available). This has caused performance and memory issues for several people trying to visit fan Ages on lower spec machines or through tools like wine.

In PlasmaMax, these load masks are handled by a "Representation Group" which allows the artist to pick different objects to render at different quality levels, but it's not especially intuitive to use. It does have the benefit of allowing some sort of association between the objects shown at different levels.

The main alternative I see is to have a "Render Quality" modifier in korman that could be attached to individual objects to control the load mask for that object in particular. This would allow for finer-grained customization, but would require the artist to keep track of which objects are rendered at which quality levels.

plLoadMask should be set on a Scene Object, and all its owned keys (ObjInterfaces, Modifiers, etc.). It is part of the key used to refer to those objects (which is how the implementation actually works: It doesn't notify with a plRefMsg if it reads a key that shouldn't load under the current quality setting).

No source files or tests are identified in the issue. Start by tracing Korman’s Scene Object export and comparing it with the PlasmaMax Representation Group behavior; the completed work should define and implement a consistent way to propagate plLoadMask to owned keys and verify rendering across quality levels.