Multisampled Bresenham lines must be rendered the same
way as non multisampled Bresenham lines, so use the
same coverage mask for all samples in that case.

Bug: b/142965928
Change-Id: Ifc0e141e4f1d9ae48c064896405164db3addd521
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38788
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>