Cleanup texture bindings after blits and resource copies and use a texture view for the blit source

#112 (closed) is the result of a texture/sampler view still being bound in one one context, but destroyed in another one, so clean up all bindings when they are no longer used. It remained that running bin/fbo-clear-formats -auto && bin/fbo-colormask-formats various times reliably crashed the VM with the given assertion failure, so as a last resort now in ...blit_int a texture view for the source texture is always created, and that seems to quell this problem.