Prepare the plumbing in the GL renderer for textures that use
target GL_TEXTURE_EXTERNAL_OES. These need to use a special sampler,
so make sure our sampler machinery does not run over it.
Instead of uploading a texture once per filter, ensure textures are
uploaded as little as possible and use samplers instead to switch
different filters.
Sometimes we have to reupload a texture unfortunately, when it is an
external one and we want to create mipmaps.
