Switch avifImage's CICP storage from enums to uint16_t, for future compatibility with amendments to H.273

Fixes: #461
diff --git a/src/write.c b/src/write.c
index b65f993..9362f52 100644
--- a/src/write.c
+++ b/src/write.c
@@ -229,9 +229,9 @@
     // Therefore, *always* writing an nclx box, even if an a prof box was already written above.
     avifBoxMarker colr = avifRWStreamWriteBox(s, "colr", AVIF_BOX_SIZE_TBD);
     avifRWStreamWriteChars(s, "nclx", 4);                                            // unsigned int(32) colour_type;
-    avifRWStreamWriteU16(s, (uint16_t)imageMetadata->colorPrimaries);                // unsigned int(16) colour_primaries;
-    avifRWStreamWriteU16(s, (uint16_t)imageMetadata->transferCharacteristics);       // unsigned int(16) transfer_characteristics;
-    avifRWStreamWriteU16(s, (uint16_t)imageMetadata->matrixCoefficients);            // unsigned int(16) matrix_coefficients;
+    avifRWStreamWriteU16(s, imageMetadata->colorPrimaries);                          // unsigned int(16) colour_primaries;
+    avifRWStreamWriteU16(s, imageMetadata->transferCharacteristics);                 // unsigned int(16) transfer_characteristics;
+    avifRWStreamWriteU16(s, imageMetadata->matrixCoefficients);                      // unsigned int(16) matrix_coefficients;
     avifRWStreamWriteU8(s, (imageMetadata->yuvRange == AVIF_RANGE_FULL) ? 0x80 : 0); // unsigned int(1) full_range_flag;
                                                                                      // unsigned int(7) reserved = 0;
     avifRWStreamFinishBox(s, colr);