Fix choosing configs with 0 alpha bits.

Fix choosing configs with 0 alpha bits.

Configs with 0 alpha bits were not being stored in the set because
the sorting functor did not compare alpha and thus did not achieve
strict ordering. Having separate functors for strict comparison and
sorting by EGL rules fixes it, and also allows to control config ID
assignment. Android expects a config with alpha bits even though not
explicitly requested.

Bug 21538709

Change-Id: I76472e0eb2a8ad9ca39c6e8ce08225d0ce5cef45
Reviewed-on: https://swiftshader-review.googlesource.com/3310
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Keun Soo Yim <yim@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>