The original code allowed Renderbuffer objects to select
layer and level at construction time, but after that point
these parameters were not modifiable. This cl adds the code
to modify layer and/or level when requested.

Change-Id: Ic694904f83b794879110bd0505a36a011d79a2a5
Reviewed-on: https://swiftshader-review.googlesource.com/4765Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>