Use empty initializer lists for zeroing structs.

This works around an incorrect Clang warning:
https://llvm.org/bugs/show_bug.cgi?id=21689

Bug 15387371

Change-Id: I2460a4f6dd414f518789be1fcc8ce8b205c6a066
Reviewed-on: https://swiftshader-review.googlesource.com/4491
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>