push to meshes only if there is sth to render

prevents "WebGL: INVALID_VALUE: vertexAttribPointer: bad size, stride or
offset lightgl.js:1059" warning when light.gl tries to render empty mesh
and size is zero.
push to meshes only if there is sth to render

prevents "WebGL: INVALID_VALUE: vertexAttribPointer: bad size, stride or
offset lightgl.js:1059" warning when light.gl tries to render empty mesh
and size is zero.
Merge remote-tracking branch 'origin/vertexAttribPointer'
Merge branch 'master' of https://github.com/wczerniak/OpenJSCAD.org