Layer visibility functions added.
Overlays list in layer switcher dialog was added.
Mid-course multi layers functionality task done. Preparing for the multi texturing planet surface.
Exceptions during  shader program variables initialization TODOs placed. Just TODOs.
Remove unused indication.
Merged shader texOffset and texScale variables to the one texBias variable.
Shader program attributes acceptance.
Added overlays functionality. But maximum overlays count is 8. It's limited by shader varying variables count.
Overlays shader program declaration changed.
Merge pull request #3 from Zemledelec/master

Multitexturing overlays. Maximum overlays is 8. Check shader's varyings.