Refactor point and line clip flag calculation.

Refactor point and line clip flag calculation.

Move clip flag calculation for the new polygon vertices of points and lines
from the renderer to the clipper.

Change-Id: I41ac3647d8e9376586a1011d1cf28d83e9c963a2
Reviewed-on: https://swiftshader-review.googlesource.com/5423
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>