Re-enable 8x8 fdct/fht tests by changing tolerance

Increases tolerance for tests to ensure the fdct tests pass after
random number range change.

Change-Id: I5b622099e850f91a14b903d10cf14685f4dd3c2b
Refactor choose_partitioning computing scheme

This commit refactors the choose_partitioning function. It removes
redundant memset calls and makes the encoder to calculate
variance value per block only when it is needed. It reduces the
average runtime cost of choose_partitioning by 60%. Overall it
reduces speed -6 runtime by 2-5%.

Change-Id: I951922c50d901d0fff77a3bafc45992179bacef9
Corrected optimization of 8x8 DCT code

The 8x8 DCT uses a fast version whenever possible.
There was a mistake in the checking code which
meant sometimes the fast version was used when it
was not safe to do so.

Change-Id: I154c84c9e2d836764768a11082947ca30f4b5ab7
(cherry picked from commit fd05fb0c21e253b4d6f92d7e0b752850ff8ab188)
Merge "Refactor choose_partitioning computing scheme"
Merge "Corrected optimization of 8x8 DCT code"
Merge "Re-enable 8x8 fdct/fht tests by changing tolerance"