Fix reading of size >4GiB on Windows.

ReadFile take a 32bits size. So we cannot blindly pass a 64bits size.
Read by batch of 4MiB.

src/fs_windows.cpp

@@ -73,22 +73,43 @@ zsize_t FD::readAt(char* dest, zsize_t size, offset_t offset) const
   EnterCriticalSection(&mp_impl->m_criticalSection);
   LARGE_INTEGER off;
   off.QuadPart = offset.v;
+  std::string errorMsg;
+  auto size_to_read = size.v;
+  size_type full_size_read = 0;
+
   if (!SetFilePointerEx(mp_impl->m_handle, off, NULL, FILE_BEGIN)) {
+	errorMsg = "Seek fail";
     goto err;
   }
 
   DWORD size_read;
-  if (!ReadFile(mp_impl->m_handle, dest, size.v, &size_read, NULL)) {
-    goto err;
+  while (size_to_read > 0) {
+	// Read by batch < 4GiB
+	// Lets use a batch of 4MiB
+	auto batch_to_read = std::min(size_to_read, (size_type)4*1024*1024);
+    if (!ReadFile(mp_impl->m_handle, dest, batch_to_read, &size_read, NULL)) {
+	  errorMsg = "Read fail";
+      goto err;
+    }
+
+	if (size_read == 0) {
+      errorMsg = "Cannot read past the end of the file";
+	  goto err;
+    }
+
+    size_to_read -= size_read;
+    full_size_read += size_read;
+	dest += size_read;
   }
-  if (size_read != size.v) {
+  if (full_size_read != size.v) {
+	errorMsg = "Wrong read size";
     goto err;
   }
   LeaveCriticalSection(&mp_impl->m_criticalSection);
   return size;
 err:
   LeaveCriticalSection(&mp_impl->m_criticalSection);
-  throw std::runtime_error("Cannot read");
+  throw std::runtime_error(errorMsg);
 }
 
 bool FD::seek(offset_t offset)