Warning from `-Wcast-qual`: cast discards `const` qualifier

[Rangi42]

In readChunk_iTXt:

error = lodepng_add_itext_sized(info, key, langtag, transkey, (char*)(data + begin), length);

data is a const char*. It happens that lodepng_add_itext_sized takes a const char* str, so this can become:

error = lodepng_add_itext_sized(info, key, langtag, transkey, (const char*)(data + begin), length);

The same (char*) to (const char*) change could be done here:

if(!error) error = lodepng_add_itext_sized(info, key, langtag, transkey, (char*)str, size);