drawSightlineDepth

Render this drawable's filled-triangle geometry into the sightline's depth pass.

Implementations are responsible for:

• binding their own vertex / element buffers,

• configuring vertex attribute 0 (the position attribute - 3 GL_FLOATs at the appropriate stride/offset),

• composing their model transform with the active sightline matrices via DrawableSightline.loadOccluderMatrix or DrawableSightline.loadOccluderTranslation immediately before each draw call,

• issuing glDrawElements / glDrawArrays.

The caller has set up exactly the state appropriate for opaque depth-only triangle rasterisation (slope-scaled polygon offset, colour writes masked). The only state the implementation typically needs to manage is cull face if its geometry isn't a closed CCW solid.

Called once per sightline face during the depth pass, so any non-trivial setup that doesn't depend on the active face matrices belongs in the caller's makeDrawable, not here.