set renderpass name in multilayer EXR

to integrate in a pipeline correctly, we need to control the name of everything precisely, and that not the case for renderlayer.

for now the prefix is the scenename, witch is not revelant in most of case as we are supposed to know what scene we are working on also the combined channel is supposed to go to "rgba"

reordering it in compositor is NOT a solution, this need to be controled before rendering I suggest that double clicking the pass allows to name it and a new textfield with a checkbox allows prefix (if wanted)