depthbuf bug fix

avoid undefined variable gl at some points
clear depthbuf correctly
during picking, use the correct viewMat and cameraMat, when there are multiple choices

NOTE: this is only guaranteed to work if there is one camera and one lookAt per layer.
Merge branch 'luke' of https://github.com/mbc/scenejs into luke

Conflicts:
	api/3.2.0/extras/gui.js
	api/3.2.0/scenejs-amd.js
	api/3.2.0/scenejs.js
	api/latest/extras/gui.js
	api/latest/scenejs-amd.js
	api/latest/scenejs.js
update build