Due to rounding used computation, HDB variance computation may produce
slightly negative values. This commit adds clamping to make sure
output variance values for 10 and 12 to be non-negative.

Change-Id: Id679aa55a4c201958c4c7d28cd8733b9246a71c8

This commit change to use int64_t to represent the sum of pixel
differences, which can be negative.

This fixes a number of ubsan warnings.

BUG=webm:1219

Change-Id: I885f245ae895ab92ca5f3b9848d37024b07aac98

This reverts commit ea48370a, reversing
changes made to 15939cb2.

The commit was insufficiently tested and causes failures.

Change-Id: I623d6fc2cd3ae6fd42d0abab1f8eada465ae57a7

Always round sum error and sum square error toward zero in variance
calculations. This prevents variance from becoming negative.
Avoiding rounding variance at all might be better but would be far
more invasive.

Change-Id: Icf24e0e75ff94952fc026ba6a4d26adf8d373f1c

Change-Id: I66bf6720c396c89aa2d1fd26d5d52bf5d5e3dff1

subpel functions will be moved in another patch.

Change-Id: Idb2e049bad0b9b32ac42cc7731cd6903de2826ce