When dealing with structs the type manager merge two different structs
into a single entry if they have all of the same decorations and
element types. This is because they hash to the same value in the hash
table. This can cause problems if you need to get the id of a type from
the type manager because you could get either one. In this case, it
returns the wrong one.

The fix avoids using the type manager in one place. I have not
looked closely at other places the type manager is used to make
sure it is used safely everywhere.

Fixes KhronosGroup#5624

This removes a use of TypeManager::GetId by keeping the id around. This
avoid a potential problem if the type manager gets confused. These types
of bugs are hard to generate test cases for, so I do not have a test.
However, existing tests make sure that do not regress.