fix: correctly map selected layers to simulcast layers

[holzgeist]

this partly fixes #133. Quick summary:

• on lower resolutions, simulcast only sends 2 (or even 1) layer.
• The layers are ordered from low to high resolution (downscale factors 4, 2, 1)
• previously, when only sending 2 layers (factors 2, 1), the layer with factor 2 was mapped to the layer with factor 4, i.e. scaled down again)
• with this patch, the layers are mapped to the correct factor and not scaled down again