Fix several issues in optical flow

Lester Lurequested to merge
kslu/avm:opflall-fix into main

This CL fixes the following issues in optical flow refinement. It also includes cleanups and refactoring of code to improve readability.

  1. In REFINE_ALL mode (--enable-opfl-refine=2), GLOBAL_GLOBALMV mode search was turned off by mistake.
  2. In REFINE_ALL mode (--enable-opfl-refine=2), interpolation filter type is implicitly derived when optical flow is used, but the condition for signaling interpolation filter type and optical flow are not always aligned (issue #274 (closed)).
  3. When reading the frame level opfl_refine_type, throw an error if the read flag is invalid (issue #257 (closed))
  4. Revert part of the OPFL baseline change, and fix an int16 MV overflow (issue #225 (closed))
  5. Fix integer MV overflow (issue #272 (closed))

Rare and minor stats changes from change 4 and 5.

Edited by Lester Lu

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