Allow a layer to implement extensions the driver doesn't #134
Description
The layer currently won't export functions that the driver doesn't support, but it would be useful to allow the layer to implement some functions that it can consume locally without forwarding, such as debug labelling.
Recent Arm GPUs support this in the drivers, but older ones do not and it would be useful to support those in the profiling layer.
Status update: Extension string exposure is now implemented as part of VK_EXT_frame_boundary support. However, it is not yet possible for a layer to expose a function that the driver does not.