The VAR_TEX definition in ISA.xml only has a field for texture_index, so trying to read sampler_index will return zero; read from texture_index instead, and rename other fields for consistency.
The texture and sampler indices must be equal for VAR_TEX to be used, so either name could be used for the field.
Fixes the wrong textures being used in Thief.
Closes: #6219 (closed) Fixes: eb1479bd ("pan/bi: Support message preloading")