Support standard simulcast with `requested_resolution`.

According to spec, if you ask for three encodings you get three
encodings (duh). But according to legacy code, if you ask for three
encodings AND codec is VP9, then surely you meant a single encoding that
is kSVC where the other encodings influence the scalability mode of the
first encoding.

Standard simulcast support in VP9 was shipped as an opt-in feature where
you have to specify `scalability_mode` and `scale_resolution_down_by` in
order to let WebRTC know that you want to disable the legacy path.

But `scale_resolution_down_by` is not the only way to configure
resolution, there is also the `requested_resolution` code path. This CL
adds standard simulcast support for this code path as well.

Prior to this change, our parameterized test would have passed in VP8
but failed in VP9. With this change the test passes for all codecs.

Bug: webrtc:361124448
Change-Id: Ic5a7136de8abf430813fd01342862775fca145fb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/360100
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42822}