The 'top' level is the last valid mipmap level nearest to the top of
the pyramid. Start searching for it from the 'base' of the pyramid.

Change-Id: I2d2ea3a1834f6cff2e2d342876d0649205004581
Reviewed-on: https://swiftshader-review.googlesource.com/15348Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>