replace reinterpret_cast with copy_n

On mips64, this throws a cast-align warning. Silence it.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/pgfimage.cpp

@@ -179,7 +179,7 @@ void PgfImage::doWriteMetadata(BasicIo& outIo) {
   // Write new Header size.
   auto newHeaderSize = static_cast<uint32_t>(header.size() + imgSize);
   DataBuf buffer(4);
-  *reinterpret_cast<uint32_t*>(buffer.data()) = newHeaderSize;
+  std::copy_n(&newHeaderSize, sizeof(uint32_t), buffer.data());
   byteSwap_(buffer, 0, bSwap_);
   if (outIo.write(buffer.c_data(), 4) != 4)
     throw Error(ErrorCode::kerImageWriteFailed);