[Discussion] Add "Driver" layers

[charles-lunarg]

Currently, there are explicit and implicit layers. The current layer ordering is to make it App->implicit->explicit->driver.

But certain layers that wish to 'extend' the capabilities of the driver will be put in the wrong spot of this chain.

One solution is to introduce a new layer type, "Driver" layer. They act and behave like implicit layers, as they would always be enabled but are loaded as close to the driver as is possible.

Related issue #328

[charles-lunarg]

From further discussions about implicit layer ordering, I do not believe that there is a strong reason for this feature to exist. As such, I'm closing the issue.