Optimize initialization of residual array
Optimize initialization of residual array

This initialization was a hotspot detected in profiling.
This change avoids initializing the unused parts of the array,
and avoids re-initializing the used parts since they will be set
anyway via the call to `diff`.

Provides approximately 3% speedup on 8-bit and 1% speedup on 10-bit.

xiph/rav1e,xiph/rav1e