Tune variance partition thresholds in RTC

[jianj-g]

Comprehensive overhaul of variance-based partition (VBP) logic and
thresholds for real-time (RTC) encoding mode across resolutions
(low-res, 360p, VGA, 720p, 1080p+) and content types (camera video
and screen content). This only affects RTC mode with speed >= 6.

1. Exponential quantizer modeling:
   Update threshold_base calculation to properly model AV2's exponential
   quantizer step size across QP, replacing the linear AC quantizer scale.

2. Resolution-adaptive thresholds and high-QP scaling:
   Refine thresholds for <= 288p, 360p, 720p, and 1080p+, introducing
   smooth high-QP threshold slopes to avoid over-partitioning fine textures
   and UI elements at coarse quantizers.

3. Sub-block variance spread evaluation & partition gating:

   • Evaluate sub-block variance spread in get_part_eval_based_on_sub_blk_var
     to avoid unnecessary 8x8 splits on flat/uniform areas while preserving
     splits for fine details.
   • Extend 32x32 sub-block variance difference checking to non-screen
     resolutions > 360p (VGA, 720p) guarded by max sub-block variance,
     avoiding trapped texture details in 32x32 blocks.
   • Guard 64x64 block splitting based on maximum sub-block variance.

4. Screen content tuning (is_screen):

   • Guard 32x32 variance-diff force-split with !is_screen to retain large
     32x32 and 16x16 blocks needed for Palette Mode and IntraBC efficiency.
   • Tailor sub-block split thresholds and high-QP scaling specifically for
     screen content.

Benchmark results vs upstream baseline:

rtc (28 clips, VGA/720p camera video):
vmaf: -5.3% | ovr_psnr: -3.7% | speedup: +4.8%

rtc_derf (20 clips, <= 288p low-res):
vmaf: -3.6% | ovr_psnr: -2.1% | speedup: +1.1%

rtc_screen (9 clips, screen content):
vmaf: -8.4% | ovr_psnr: -3.3% | speedup: +2.2%

STATS_CHANGED for RTC mode