* Reduce bit widths of intermediate values where possible
* Change ROUND_POWER_OF_TWO_SIGNED to ROUND_POWER_OF_TWO
  in av1(_highbd)_convolve_2d
* Apply offsetting and bounds checking, to match the intended
  hardware implementation
* Separate the implementations of av1(_highbd)_convolve_2d
  into compound-round and non-compound-round cases. This is because
  there are now a significant number of differences between the
  functions.

Overall, this is expected to affect the bitstream and encoder output
when convolve-round alone is enabled, but *not* when compound-round
is enabled.

Change-Id: I8c21e0645fd11f64c59552885f87f4a5dd40ccf7