Pass voxels in emitted 'meshed' event
Change createVoxelMesh() to not return null for no vertices

Instead, it returns an object with triangleVertexCount=0 and
triangleVAO=null. The other fields are populated. This allows other
meshers (such as voxel-wireframe, but more importantly a future chunk
boundary debug tool https://github.com/deathcap/voxel-ideas/issues/14)
to add their own meshes to the result, even if there are no vertices
from us (for example, if the entire chunk is air).

Requires deathcap/voxel-shader@726fbcc1a7ad6080fd3a9f483b9b510218d7a392
0.9.0
Move triangleVertexCount to triangleVAO.length