Wire `sharpness` value to loop filter sharpness in allintra mode

Currently, loop filter sharpness is always set to 0, which means
`sharpness` values from 1 to 7 are effectively equivalent.

Setting loop filter sharpness helps the SSIMU2 88-93 average range,
and improves the SSIMU2 0-75 10th percentile on Daala's subset1.

Bug: b:374821854
Change-Id: Iae018161de01752dfce3b52369d0bf883e7120c7

av1/encoder/picklpf.c

diff --git a/av1/encoder/picklpf.c b/av1/encoder/picklpf.c
index 0851d97..36af32a 100644
--- a/av1/encoder/picklpf.c
+++ b/av1/encoder/picklpf.c
@@ -215,7 +215,10 @@
   int disable_filter_rt_screen = 0;
   (void)sd;
 
-  lf->sharpness_level = 0;
+  // Enable loop filter sharpness only for allintra encoding mode,
+  // as frames do not have to serve as references to others
+  lf->sharpness_level =
+      cpi->oxcf.mode == ALLINTRA ? cpi->oxcf.algo_cfg.sharpness : 0;
 
   if (cpi->oxcf.tune_cfg.content == AOM_CONTENT_SCREEN &&
       cpi->oxcf.q_cfg.aq_mode == CYCLIC_REFRESH_AQ &&