Pack texelFetch LOD as sampling coordinate's w component.

Pack texelFetch LOD as sampling coordinate's w component.

One less argument is emitted.

Vertex/PixelProgram sampleTexture function signature simplified.

Change-Id: I7aef3eb100ccb51a8bd9d5fd600c73b4843d30d9
Reviewed-on: https://swiftshader-review.googlesource.com/5600
Tested-by: Meng-Lin Wu <marleymoo@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>