Add support to share a single GL context between layers

Share a single viewer between all vglRenderer instances so they
can share the same GL canvas.
Move VGL submodule forward
Only register events for base layer
Fix style issues
Merge remote-tracking branch 'origin/shared-gl-context'

Conflicts:
	src/gl/vglRenderer.js
Fixed missing var initialization
Removed default renderer as it may cause bad behavior
Merge pull request #95 from OpenGeoscience/shared-gl-context

Shared gl context