Fix IVAS_rend usage as post renderer in BS.1534
BS.1534 configs incorrectly use the key rend which should be ivas_rend.
This causes python rendering to unintentionally be used for upmixing EVS Mono to Stereo.
For specifically BS.1534-19, this was missing for performing the static downmix of Dual EVS output to Mono.
Edited by Archit Tamarapu