Previously the rasterizer was just using sample location 0. The Vulkan
1.1.130 spec states that "When VK_LINE_RASTERIZATION_MODE_BRESENHAM_EXT
lines are being rasterized, sample locations may all be treated as being
at the pixel center". And also "Do we need to support Bresenham-style
and smooth lines with more than one rasterization sample? RESOLVED: Yes.
For simplicity, Bresenham line rasterization carries forward a few
restrictions from OpenGL, such as not supporting per-sample shading,
alpha to coverage, or alpha to one."

Bug: b/142965928
Change-Id: I586ffbaa92d506595d6fecfa2dbd64f10bdee0c8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40188
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>