Fix build error with msvc x86

diff --git a/src/read.c b/src/read.c
index 696188d..63989f9 100644
--- a/src/read.c
+++ b/src/read.c
@@ -950,7 +950,7 @@
                     return AVIF_RESULT_BMFF_PARSE_FAILED;
                 }
                 startOffset += remainingOffset;
-                extentSize -= remainingOffset;
+                extentSize -= (size_t)remainingOffset;
                 remainingOffset = 0;
             }
         }