VulkanVideoParser: preserve DPB slots for key frames with partial refresh

For AV1 key frames with show_frame=0 and refresh_frame_flags != 0xFF,
preserve DPB slots that are not being refreshed. This allows subsequent
inter frames to correctly reference existing decoded pictures that were
not overwritten by the hidden key frame.

Previously, all DPB slots could be invalidated on key frames, breaking
references for inter frames following a key frame with partial
refresh flags and show_frame=0.

Author: crmurillo

vk_video_decoder/libs/VkVideoParser/VulkanVideoParser.cpp

@@ -1848,6 +1848,23 @@ uint32_t VulkanVideoParser::FillDpbAV1State(
         printf("\n");
     }
 
+    // For key frames with show_frame=0 and refresh_frame_flags!=0xFF,
+    // DPB slots not specified in the flags should remain valid so subsequent inter frames can reference them.
+    if (isKeyFrame && !pin->showFrame && pin->std_info.refresh_frame_flags != 0xFF) {
+        uint8_t preserveMask = ~pin->std_info.refresh_frame_flags;
+        for (uint32_t inIdx = 0; inIdx < STD_VIDEO_AV1_NUM_REF_FRAMES; inIdx++) {
+            if (preserveMask & (1 << inIdx)) {
+                int8_t picIdx = pin->pic_idx[inIdx];
+                if (picIdx >= 0) {
+                    refDpbUsedAndValidMask |= (1 << picIdx);
+                    if (m_dumpDpbData) {
+                        printf("Preserving DPB slot %d for picIdx %d\n", inIdx, picIdx);
+                    }
+                }
+            }
+        }
+    }
+
     ResetPicDpbSlots(refDpbUsedAndValidMask);
 
     // Take into account the reference picture now.