leadingVertexFirst is used within rendering threads and assigned in
the Renderer's constructor, so TSAN detects this as a data race. In
order to fix this, we can make sure any of these global rendering
settings are only set once (since they never change).

Change-Id: Ib4b43d181b140e500ca31e1ce48340c7f4ce500d
Reviewed-on: https://swiftshader-review.googlesource.com/20008Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>