Reorganize code to faciliate setting rounding parameters based
on bit-depth, and to faciliate testing.

After this patch this wil be the behavior for config flags as far
as round_0 and round_1 choices are concerned for 8- and 10-bit:

0. CONFIG_LOWPRECISION_BLEND=0 CONFIG_HIGHPRECISION_INTBUF=0:
round_0 = 5, round_1 = None (baseline)

1. CONFIG_LOWPRECISION_BLEND=0 CONFIG_HIGHPRECISION_INTBUF=1:
round_0 = 3, round_1 = None (to test impact of increase in precision
of intermediate buffer)

2. CONFIG_LOWPRECISION_BLEND=1 CONFIG_HIGHPRECISION_INTBUF=0:
round_0 = 5, round_1 = 4

3. CONFIG_LOWPRECISION_BLEND=2 CONFIG_HIGHPRECISION_INTBUF=0:
round_0 = 5, round_1 = 5

4. CONFIG_LOWPRECISION_BLEND=1 CONFIG_HIGHPRECISION_INTBUF=1:
round_0 = 3, round_1 = 6 (ARM proposal except clipping)

5. CONFIG_LOWPRECISION_BLEND=2 CONFIG_HIGHPRECISION_INTBUF=1:
round_0 = 3, round_1 = 7 (Google variation proposal)

Change-Id: I615348332f5692135352085ca923662f9d52f696