Lower threshold to skip YUV-RGB conversion tests

Lower the threshold for skipping the YUV<->RGB conversion tests from
35 * 1024 * 1024 to 30 * 1024 * 1024.

A test image of size 11011 x 3328 causes a sanitizer build to spend
18s in avifImageYUVToRGB() and 13s in avifImageRGBToYUV().

Bug: b/296560073

[skip ci]

tests/oss-fuzz/avif_decode_fuzzer.cc

diff --git a/tests/oss-fuzz/avif_decode_fuzzer.cc b/tests/oss-fuzz/avif_decode_fuzzer.cc
index 117aae5..3081171 100644
--- a/tests/oss-fuzz/avif_decode_fuzzer.cc
+++ b/tests/oss-fuzz/avif_decode_fuzzer.cc
@@ -32,7 +32,7 @@
     if (result == AVIF_RESULT_OK) {
         for (int loop = 0; loop < 2; ++loop) {
             while (avifDecoderNextImage(decoder) == AVIF_RESULT_OK) {
-                if (((decoder->image->width * decoder->image->height) > (35 * 1024 * 1024)) || (loop != 0) ||
+                if (((decoder->image->width * decoder->image->height) > (30 * 1024 * 1024)) || (loop != 0) ||
                     (decoder->imageIndex != 0)) {
                     // Skip the YUV<->RGB conversion tests, which are time-consuming for large
                     // images. It suffices to run these tests only for loop == 0 and only for the