svga: tweak pre-VGPU10 rasterization offsets

svga: tweak pre-VGPU10 rasterization offsets

It seems there's no perfect x/y biases for line drawing to satisfy all
applications.  Depending on the biases, either real apps produce results
similar to VGPU10 while Piglit's gl-1.0-ortho-pos fails, or vice versa.

Let's lean toward real applications (Solidworks, SolidEdge, Google Earth)
over Piglit.

Using (-0.5, -0.5) for points, lines and triangles, seems to generally
work well.

We don't seem to have these issues with VGPU10.

Tested with Piglit and CAD-oriented apitraces.  See VMware bugs 1775498
and 1905053.

Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>