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

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