Add superblock source SAD speed feature for RTC partitioning

[jianj-g]

Computes superblock source SAD (between current source and collocated previous source block) and incorporates it into variance-based partitioning (VBP) for real-time (RTC) encoding mode (speed >= 6).

Key changes:

1. Superblock source SAD metrics:

   • Added SOURCE_SAD classification enum (kZeroSad, kVeryLowSad, kLowSad, kMedSad, kHighSad) and tracking fields in MACROBLOCK.
   • Gated under sf->rt_sf.source_metrics_sb (enabled at speed >= 6).
   • Computed per-SB source SAD in encode_rd_sb via cpi->fn_ptr[sb_size].sdf and normalized to 64x64 scale.

2. Variance-based partition tuning:

   • Early exit fast path for static superblocks (source_sad == 0 && y_sad == 0), directly selecting the whole superblock size without evaluating sub-block variance quadtrees.
   • Scaled partition split thresholds (thresholds[1]..thresholds[4]) for low-motion superblocks (source_sad_level <= kLowSad) to favor larger blocks.
   • Adjusted low-resolution QP transition bounds in tune_thresh_based_on_resolution when source_sad_level <= kLowSad.
   • Guarded 16x16 and 32x32 variance-difference forced splits with (source_sad_level > kLowSad).

STATS_CHANGED for RTC mode

TAG=agy
CONV=9425edb0-f25c-4ea7-b0c5-9c5c7ed895c0