op: Allow bit patterns for the same value in copyTextureToTexture

[Kangz]

For example 0x80 and 0x81 both represent -1 in r8snorm and it should be valid for B->T->T->B to convert one to the other.

[kainino0x]

update: The texture_ok helpers account for this. I'm not sure if these tests use them yet, but probably not? If we update them to use texture_ok, then it should be fixed.