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
This is based on spv.paramMemory.frag.out which exercises the
writeonly storage image case.
This appears to need desktop GLSL.
The generated SPIR-V fails validation because the image_write function
takes a parameter which is pointer to an OpTypeImage with Unknown format.
But the parameters passed in are pointer to OpTypeImage with formats Rgba32f
and Rgba16f. The validator rejects this, saying the parameter types must
match.
