This enables binding any texture mipmap level as a framebuffer
attachment.

Bug swiftshader:104

Change-Id: I3d4ea637ddd38bb62ca1363fe2c69c569eea36e9
Reviewed-on: https://swiftshader-review.googlesource.com/18008Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Lingfeng Yang <lfy@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>