texture binding

[adam-ce]

it seems i misunderstood textures a bit. we need to bind before upload. otherwise the target will switch. i guess it's similar with texture properties. they likely attach to the texture unit and not the texture. i think we should protect against that.

• better understand the state machine
• implement a protection, e.g.: texture unit in constructor + static active texture unit. if the active unit is not same as own unit, activate first.