In imageioJPEG.c, we read one scanline at a time, then copy the decoded data into
a temporary buffer before stuffing it into the destination raster. That intermediate
copy step is currently not as efficient as possible since it copies one byte at a
time, but in the common case (where jpeg_read_scanlines() produces 3ByteRgb data)
the component data is already ordered, so we could simply use memcpy() instead.