lowres: bug fix -  move initialization from create() to init()

lowres objects are reused, hence they should be re-initialized.

--HG--
branch : stable
Adapting lowress frame dimensions to multiples of lowress CU size.

The full res frame is required to be extended accordingly to achieve a lowres frame with dimensions in multiples of lowres CU size.

--HG--
branch : stable
Merge with stable
lowres: white-space nits and minor cleanup
lowres: remove NULL pointer checks, x265_free does this internally
Replace combo padd(32)+psra(6) by pmulhrsw
Replace (maskmovq) by (blendvb + movq)
encoder: explicitly disable AMP if rectangular modes are disabled

In the mode decision logic, asymmetrical motion prediction is implicitly
disabled when rectangular modes are disabled. This makes the disabling more
explicit, AMP will not appear as enabled in the "encoding tools" log line.
lowres: fixup bad merge/patch re-import