When a sampler object is bound to a texture unit, it overrides the
texture's filter modes. A texture without mipmaps can thus be complete
even if it has a filter mode which requires mipmaps, if there's a
sampler object with a filter mode which doesn't require mipmaps.

Fixes https://github.com/google/filament/pull/220

Change-Id: I200c0bba467d904ac0fd6d93b935e052c47b2030
Reviewed-on: https://swiftshader-review.googlesource.com/20529Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Jeff McGlynn <jwmcglynn@google.com>
Tested-by: Jeff McGlynn <jwmcglynn@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>