Set some core specific values

[gutomaia]

I've set some core specific values. for testing. Example using the snes9x core, i'm using setting the env_vars values to disable same layers from rendering.

emu.env_vars[b'snes9x_layer_1'] = b'disable'

First, trying to disable some noticeable features on the core, although it's not working.

[fregapple]

When I was looking at this project, I did find a way to change the core values. Though can't exactly remember what I did.

I think I wasn't able to change the values while the core was loaded. So I had to set them during core initialisation.