They're big and take a while to initialize. Only construct them if we need them.
This also pushes the construction onto the queue, speeding up the main thread.

Bug: b/139010488
Change-Id: I2d1d25f24162339de62cea4d946532b1a2e346ca
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34330Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>