tools/target_dec_fuzzer: Adjust threshold for PNG and APNG

Fixes: Timeout (84sec -> 2sec)
Fixes: 21127/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APNG_fuzzer-5098412367413248

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

tools/target_dec_fuzzer.c

@@ -157,6 +157,8 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     case AV_CODEC_ID_MSRLE:       maxpixels  /= 16;    break;
     case AV_CODEC_ID_MSS2:        maxpixels  /= 16384; break;
     case AV_CODEC_ID_MSZH:        maxpixels  /= 128;   break;
+    case AV_CODEC_ID_PNG:         maxpixels  /= 128;   break;
+    case AV_CODEC_ID_APNG:        maxpixels  /= 128;   break;
     case AV_CODEC_ID_QTRLE:       maxpixels  /= 16;    break;
     case AV_CODEC_ID_RASC:        maxpixels  /= 16;    break;
     case AV_CODEC_ID_SANM:        maxpixels  /= 16;    break;