Experimental support for enabling the use of "surface input" mode

even with the software VP8 encoder.
This relies heavily on the fact that the "Nexus" devices use ARGB32 as
the colorspace for the data underlying a surface provided by SurfaceFlinger
(mirroring). Generally there are no such guarantees.

Change-Id: I1de32f591a3bb935ca76151816b3a02665bec40b