Fix overflows in two functions in src/reformat.c

Fix overflows when multiplying with rowBytes in avifImageRGBToYUV() and
avifImageYUVAnyToRGBAnySlow(), by storing the various uint32_t rowBytes
fields in local variables of the size_t type. Then multiplications with
the size_t rowBytes local variables will be done in size_t.

Part of the fix to #2271.

src/reformat.c

@@ -286,8 +286,8 @@ avifResult avifImageRGBToYUV(avifImage * image, const avifRGBImage * rgb)
                 // Convert an entire 2x2 block to YUV, and populate any fully sampled channels as we go
                 for (int bJ = 0; bJ < blockH; ++bJ) {
                     for (int bI = 0; bI < blockW; ++bI) {
-                        int i = outerI + bI;
-                        int j = outerJ + bJ;
+                        size_t i = outerI + bI;
+                        size_t j = outerJ + bJ;
 
                         // Unpack RGB into normalized float
                         if (state.rgb.channelBytes > 1) {
@@ -420,8 +420,8 @@ avifResult avifImageRGBToYUV(avifImage * image, const avifRGBImage * rgb)
 
                     const int chromaShiftX = 1;
                     const int chromaShiftY = 1;
-                    int uvI = outerI >> chromaShiftX;
-                    int uvJ = outerJ >> chromaShiftY;
+                    size_t uvI = outerI >> chromaShiftX;
+                    size_t uvJ = outerJ >> chromaShiftY;
                     if (state.yuv.channelBytes > 1) {
                         uint16_t * pU = (uint16_t *)&yuvPlanes[AVIF_CHAN_U][(uvI * 2) + (uvJ * yuvRowBytes[AVIF_CHAN_U])];
                         *pU = (uint16_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);
@@ -448,8 +448,8 @@ avifResult avifImageRGBToYUV(avifImage * image, const avifRGBImage * rgb)
                         float avgV = sumV / totalSamples;
 
                         const int chromaShiftX = 1;
-                        int uvI = outerI >> chromaShiftX;
-                        int uvJ = outerJ + bJ;
+                        size_t uvI = outerI >> chromaShiftX;
+                        size_t uvJ = outerJ + bJ;
                         if (state.yuv.channelBytes > 1) {
                             uint16_t * pU = (uint16_t *)&yuvPlanes[AVIF_CHAN_U][(uvI * 2) + (uvJ * yuvRowBytes[AVIF_CHAN_U])];
                             *pU = (uint16_t)avifYUVColorSpaceInfoUVToUNorm(&state.yuv, avgU);