We can avoid adding the base level to the computed lod by putting the
base level at index 0 of the mipmap array.

Change-Id: I0bdebec3e4d0a3617dea9e3d28fdeb5dae40444b
Reviewed-on: https://swiftshader-review.googlesource.com/15329Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>