video: Add support for GRAY12_PACKED and GRAY12_LE16

This adds support for packed and unpacked 12 bits grayscale video
formats. These formats are used by some Raptor SWIR cameras and other
Cameralink cameras.

GRAY12_LE16: 12 bits pixels stored on 16 bits (with 4 bits of padding).
For an image of 2x2 pixels, containing pixels AAA, BBB, CCC and DDD,
it is encoded in memory as 8 bytes:

0A AA 0B BB 0C CC 0D DD

GRAY12_PACKED: 12 bits pixels with no padding. 2 pixels packed in 3 bytes.
For an image of 2x2 pixels, containing pixels AAA, BBB, CCC and DDD,
it is encoded in memory as 6 bytes:

AA AB BB CC CD DD