kwinglutils: Make GLRenderTarget not store GLTexture reference

This allows us to make the GLRenderTarget a bit nicer when using it to
wrap the default fbo as we don't know what the color attachment texture
is besides its size.

This means that the responsibility of ensuring that the color attachment
outlives the fbo is now up to the caller. However, most of kwin code
has been written that way, so it's not an issue.