Fail on reformat Identity with subsampling, make avifenc correct this before reformat (#517)

According to [AV1 Spec](https://aomediacodec.github.io/av1-spec/#color-config-semantics), Identity is only valid with YUV444:
> If matrix_coefficients is equal to MC_IDENTITY, it is a requirement of bitstream conformance that subsampling_x is equal to 0 and subsampling_y is equal to 0.

But `avifImageRGBToYUV` still tries to do the reformat. avifenc tries to correct this, but after reformat, which will produce an valid image but with wrong color.

This PR lets `avifImageRGBToYUV` failed on `image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY && image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444` so that libavif won't produce corrupted file. This PR also makes avifenc correct MC before reformat so that the output image has correct color.

I'm still thinking should we change MC or yuvFormat in this case? If user wants lossless, then maybe we should change yuvFormat instead, which makes output lossless as requested.

By the way, if we try to make an Identity monochrome image (forbid by spec), libavif, Chrome and Firefox all wouldn't reject decoding it. However libavif and Firefox will copy the Y(G) value to U(B) and V(R), but Chrome will fill U(B) and V(R) with 128.

apps/avifenc.c

diff --git a/apps/avifenc.c b/apps/avifenc.c
index b4ba238..7370661 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -728,6 +728,18 @@
     image->yuvRange = requestedRange;
     image->alphaPremultiplied = premultiplyAlpha;
 
+    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (input.requestedFormat != AVIF_PIXEL_FORMAT_YUV444)) {
+        // matrixCoefficients was likely set to AVIF_MATRIX_COEFFICIENTS_IDENTITY as a side effect
+        // of --lossless, and Identity is only valid with YUV444. Set this back to the default.
+        image->matrixCoefficients = AVIF_MATRIX_COEFFICIENTS_BT601;
+
+        if (cicpExplicitlySet) {
+            // Only warn if someone explicitly asked for identity.
+            printf("WARNING: matrixCoefficients may not be set to identity (0) when subsampling. Resetting MC to defaults (%d).\n",
+                   image->matrixCoefficients);
+        }
+    }
+
     avifInputFile * firstFile = avifInputGetNextFile(&input);
     uint32_t sourceDepth = 0;
     avifAppSourceTiming firstSourceTiming;
@@ -739,6 +751,13 @@
     }
     avifBool sourceWasRGB = (inputFormat != AVIF_APP_FILE_FORMAT_Y4M);
 
+    // Check again for y4m input (y4m input ignores input.requestedFormat and retains the format in file).
+    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444)) {
+        fprintf(stderr, "matrixCoefficients may not be set to identity (0) when subsampling.\n");
+        returnCode = 1;
+        goto cleanup;
+    }
+
     printf("Successfully loaded: %s\n", firstFile->filename);
 
     // Prepare image timings
@@ -756,18 +775,6 @@
         }
     }
 
-    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444)) {
-        // matrixCoefficients was likely set to AVIF_MATRIX_COEFFICIENTS_IDENTITY as a side effect
-        // of --lossless, and Identity is only valid with YUV444. Set this back to the default.
-        image->matrixCoefficients = AVIF_MATRIX_COEFFICIENTS_BT601;
-
-        if (cicpExplicitlySet) {
-            // Only warn if someone explicitly asked for identity.
-            printf("WARNING: matrixCoefficients may not be set to identity (0) when subsampling. Resetting MC to defaults (%d).\n",
-                   image->matrixCoefficients);
-        }
-    }
-
     if (ignoreICC) {
         avifImageSetProfileICC(image, NULL, 0);
     }

src/reformat.c

diff --git a/src/reformat.c b/src/reformat.c
index be035c4..e318487 100644
--- a/src/reformat.c
+++ b/src/reformat.c
@@ -36,9 +36,14 @@
         return AVIF_FALSE;
     }
 
+    if ((image->matrixCoefficients == AVIF_MATRIX_COEFFICIENTS_IDENTITY) && (image->yuvFormat != AVIF_PIXEL_FORMAT_YUV444)) {
+        return AVIF_FALSE;
+    }
+
     if (image->yuvFormat == AVIF_PIXEL_FORMAT_NONE) {
         return AVIF_FALSE;
     }
+
     avifGetPixelFormatInfo(image->yuvFormat, &state->formatInfo);
     avifCalcYUVCoefficients(image, &state->kr, &state->kg, &state->kb);
     state->mode = AVIF_REFORMAT_MODE_YUV_COEFFICIENTS;