sightlineCubeFramebuffer

Depth-only framebuffer paired with sightlineDepthCubeTexture. The depth pass re-attaches the face being rendered to GL_DEPTH_ATTACHMENT via Framebuffer.attachTexture with the matching GL_TEXTURE_CUBE_MAP_* target. With no colour attachment, desktop GL reports FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER unless the draw and read buffers are explicitly set to NONE (FBO state, set once here).