Problem of supporting builtin blocks boils down to how you refer to them
from the API side -- if the reference is ONLY a SPIRV id, then block
members are difficult to work with. Instead, represent a builtin
variable binding as (id, first component, num components). For a single
variable, first component == 0 always.

Bug: b/120799499
Change-Id: If9e99f4c10f73a008e2f5071a95785920c7fbed1
Reviewed-on: https://swiftshader-review.googlesource.com/c/23488Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>