If all subencoders support textures, the adapter will claim support.
Texture frames will be passed on directly to subencoders, without any
attempt at scaling, and subencoders will be expected to sample/scale
correctly from source textures.
BUG=
NOTRY=true
Review-Url: https://codereview.webrtc.org/2099483002
Cr-Commit-Position: refs/heads/master@{#13365}