Check tiles in DecRowCount() and not contentToDec (#3012)

Update avifDecoderDecodedRowCount() description.
Remove experimental warning.

include/avif/avif.h

diff --git a/include/avif/avif.h b/include/avif/avif.h
index 8eb84f5..204a96e 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -1441,17 +1441,16 @@
 // function can be called next to retrieve the number of top rows that can be immediately accessed
 // from the luma plane of decoder->image, and alpha if any. The corresponding rows from the chroma planes,
 // if any, can also be accessed (half rounded up if subsampled, same number of rows otherwise).
-// If a gain map is present and  (imageContentToDecode & AVIF_IMAGE_CONTENT_GAIN_MAP) is nonzero,
-// the gain map's planes can also be accessed in the same way.
-// If the gain map's height is different from the main image, then the number of available gain map
-// rows is at least:
-// roundf((float)decoded_row_count / decoder->image->height * decoder->image->gainMap.image->height)
+// If a gain map is present and (imageContentToDecode & AVIF_IMAGE_CONTENT_GAIN_MAP) was nonzero when
+// avifDecoderNextImage() or avifDecoderNthImage() was called, the gain map's planes can also be accessed
+// in the same way. If the gain map's height is different from the main image, then the number of
+// available gain map rows is at least:
+//   roundf((float)decoded_row_count / decoder->image->height * decoder->image->gainMap.image->height)
 // When gain map scaling is needed, callers might choose to use a few less rows depending on how many rows
 // are needed by the scaling algorithm, to avoid the last row(s) changing when more data becomes available.
 // decoder->allowIncremental must be set to true before calling avifDecoderNextImage() or
 // avifDecoderNthImage(). Returns decoder->image->height when the last call to avifDecoderNextImage() or
 // avifDecoderNthImage() returned AVIF_RESULT_OK. Returns 0 in all other cases.
-// WARNING: Experimental feature.
 AVIF_API uint32_t avifDecoderDecodedRowCount(const avifDecoder * decoder);
 
 // ---------------------------------------------------------------------------

src/read.c

diff --git a/src/read.c b/src/read.c
index 8f22250..882be17 100644
--- a/src/read.c
+++ b/src/read.c
@@ -7140,7 +7140,7 @@
     for (int c = 0; c < AVIF_ITEM_CATEGORY_COUNT; ++c) {
         if (c == AVIF_ITEM_GAIN_MAP) {
             const avifImage * const gainMap = decoder->image->gainMap ? decoder->image->gainMap->image : NULL;
-            if ((decoder->imageContentToDecode & AVIF_IMAGE_CONTENT_GAIN_MAP) && gainMap != NULL && gainMap->height != 0) {
+            if (gainMap != NULL && gainMap->height != 0 && decoder->data->tileInfos[AVIF_ITEM_GAIN_MAP].tileCount != 0) {
                 uint32_t gainMapRowCount = avifGetDecodedRowCount(decoder, &decoder->data->tileInfos[AVIF_ITEM_GAIN_MAP], gainMap);
                 if (gainMap->height != decoder->image->height) {
                     const uint32_t scaledGainMapRowCount =