It is invalid if the same decoration is applied to the same id multiple
times. This adds a check before adding a decoration that the decoration
is not already in the list. If it is, then the duplicate is not added.
Fixes#3627
Without this commit, if the XfbStride was explicitly set, the
decoration was added twice on the shader.
v2 (changes after Jonh Kessenich first review)
* Simplified by just removing the firs assignment
* Removed assert
