Renamed Vector4i to Vector4s

Renamed Vector4i to Vector4s

The current Vector4i was a vector of shorts
and OpenGL ES 3.0 actually requires vectors
of ints and uints, so I renamed Vector4i to
Vector4s so that I may later add Vector4i
and Vector4u with the proper internal
representation.

Change-Id: I9d413c459154c7ef9ddafc46b9fb7fa6186633b8
Reviewed-on: https://swiftshader-review.googlesource.com/2851
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>