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 type was removed from spirv1.6. If samplerBuffer is specified in
GLSL, generate textureBuffer. If samplerBuffer type is constructed,
just return the buffer.
Fixes#2956