This lets users specify the level of sampling precision they want.
Chromium needs this to pass WebGL tests with SWANGLE.

Bug: b/146423360
Bug: b/154620295

Change-Id: I83575823b5909d836c3d4c02803ba7909ce08935
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42268Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>