It is legal for a shader to not declare BuiltInPosition.
This cl adds logic to use 0 for all position components
when the position builtin is not present and then keeps
executing the rest of the shader normally.

Bug: b/176161380
Change-Id: I1b4637b813d3d30958e2db655d299603d625c20a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51808
Commit-Queue: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>