Pure-air chunks (above terrain) and chunks where every face culls
produce a zero-length mesh from ChunkMesher.Build(). Previously
InstallResult still allocated a real ChunkRenderer (VBO + VAO) and
registered a zero-vertex draw call. With RenderDistanceVertical=2 and
a player at surface Y, the topmost vertical layer is typically all
pure air -- roughly H*H = ~289 wasted renderers per spawn.
The chunk still gets added to _ready so SpawnReady, dirty-marking and
unload bookkeeping continue to work; DrawAll's iteration over the
renderer dictionary naturally skips entries that don't exist.
MDietermann
d1c8c22705