Convert planar 8 bit formats to planar 16 bit formats.
Accepts a parameter that determines the number of bits.

Bug: libyuv:751
Test: Convert8To16 unittest
Change-Id: I8f6ffe64428ddf5769b87e0c069093a50a2541e9
Reviewed-on: https://chromium-review.googlesource.com/835410Reviewed-by: richard winterton <rrwinterton@gmail.com>