Add unbindImageTexture

The parameters `level, layered, layer, access & format` seem to be irrelevant for unbinding, according to the spec, but must be from valid ranges.

Also, remove unnecessary call to bind in bindImageTexture as the texture to bind is identified via its ID.
Well, at least its how I would interpret it...
From https://www.opengl.org/sdk/docs/man/html/glBindImageTexture.xhtml: *If texture is zero, then any existing binding to the image unit is broken.*