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
instead of allowing for multiple declarations of the variable in the
resulting SPIR-V, instead use a retargeted mechanism to cause references
to gl_VertexID and gl_InstanceID to use the gl_VertexIndex and
gl_InstanceIndex symbol.
Avoid adding global uniform blocks to stages that don't already have it.
Otherwise multiple stages point to the same block object, and a
remapping that occurs later on will change the mapping on multiple
stages.
