Improve detection of libyuv integer overflow bugs

The detection code added in
https://github.com/AOMediaCodec/libavif/pull/735 is off by one.

Also add comments to explain the detection code.

Fix https://github.com/AOMediaCodec/libavif/issues/767.

src/scale.c

diff --git a/src/scale.c b/src/scale.c
index 1ac3acb..42d9341 100644
--- a/src/scale.c
+++ b/src/scale.c
@@ -71,18 +71,31 @@
     image->height = dstHeight;
 
     if (srcYUVPlanes[0]) {
+        // Detect integer overflows in libyuv's ScalePlane() and ScalePlane_12() functions. For a
+        // representative example, see the ScalePlaneBox() function. It calls ScaleScape() to set
+        // x, y, dx, and dy. It then does "y += dy" in a loop with dst_height iterations. It may
+        // call ScaleAddCols2_C(), which does "x += dx" in a loop with dst_width iterations. We
+        // simulate the libyuv code and detect if the addition would overflow.
+        //
+        // Divide srcWidth by dstWidth and return as 16.16 fixed point result.
         const int fixedPointDivWidth = (int)(((int64_t)(srcWidth) << 16) / dstWidth);
-        const int64_t maxFixedValueWidth = (int64_t)fixedPointDivWidth * (dstWidth - 1);
+        // The maximum value of the sum when adding fixedPointDivWidth in a loop with dstWidth
+        // iterations.
+        const int64_t maxFixedValueWidth = (int64_t)fixedPointDivWidth * dstWidth;
         if (maxFixedValueWidth > INT_MAX) {
             avifDiagnosticsPrintf(diag, "avifImageScale requested invalid width scale for libyuv [%u -> %u]", srcWidth, dstWidth);
             return AVIF_FALSE;
         }
+        // Divide srcHeight by dstHeight and return as 16.16 fixed point result.
         const int fixedPointDivHeight = (int)(((int64_t)(srcHeight) << 16) / dstHeight);
-        const int64_t maxFixedValueHeight = (int64_t)fixedPointDivHeight * (dstHeight - 1);
+        // The maximum value of the sum when adding fixedPointDivHeight in a loop with dstHeight
+        // iterations.
+        const int64_t maxFixedValueHeight = (int64_t)fixedPointDivHeight * dstHeight;
         if (maxFixedValueHeight > INT_MAX) {
             avifDiagnosticsPrintf(diag, "avifImageScale requested invalid height scale for libyuv [%u -> %u]", srcHeight, dstHeight);
             return AVIF_FALSE;
         }
+
         avifImageAllocatePlanes(image, AVIF_PLANES_YUV);
 
         avifPixelFormatInfo formatInfo;
@@ -123,17 +136,18 @@
 
     if (srcAlphaPlane) {
         const int fixedPointDivWidth = (int)(((int64_t)(srcWidth) << 16) / dstWidth);
-        const int64_t maxFixedValueWidth = (int64_t)fixedPointDivWidth * (dstWidth - 1);
+        const int64_t maxFixedValueWidth = (int64_t)fixedPointDivWidth * dstWidth;
         if (maxFixedValueWidth > INT_MAX) {
             avifDiagnosticsPrintf(diag, "avifImageScale requested invalid width scale for libyuv [%u -> %u]", srcWidth, dstWidth);
             return AVIF_FALSE;
         }
         const int fixedPointDivHeight = (int)(((int64_t)(srcHeight) << 16) / dstHeight);
-        const int64_t maxFixedValueHeight = (int64_t)fixedPointDivHeight * (dstHeight - 1);
+        const int64_t maxFixedValueHeight = (int64_t)fixedPointDivHeight * dstHeight;
         if (maxFixedValueHeight > INT_MAX) {
             avifDiagnosticsPrintf(diag, "avifImageScale requested invalid height scale for libyuv [%u -> %u]", srcHeight, dstHeight);
             return AVIF_FALSE;
         }
+
         avifImageAllocatePlanes(image, AVIF_PLANES_A);
 
         if (image->depth > 8) {