i965: Fix non-AA wide line rendering with fractional line widths

i965: Fix non-AA wide line rendering with fractional line widths

"(...)Let w be the width rounded to the nearest integer (...). If the
line segment has endpoints given by (x0,y0) and (x1,y1) in window
coordinates, the segment with endpoints (x0,y0-(w-1)/2) and
(x1,y1-(w-1/2)) is rasterized, (...)"

The hardware it not rounding the line width, so we should do it.

Also, we should be careful not to go beyond the hardware limits
for the line width after it gets rounded. Gen6-7 define a maximum line
width slightly below 8.0, so we should advertise a maximum line
width lower than 7.5 to make sure that 7.0 is the maximum integer
line width that we can select. Since the line width granularity in these
platforms is 0.125, we choose 7.375. Other platforms advertise rounded
maximum line widths, so those are fine.

Fixes the following 3 dEQP tests:
dEQP-GLES3.functional.rasterization.primitives.lines_wide
dEQP-GLES3.functional.rasterization.fbo.texture_2d.primitives.lines_wide
dEQP-GLES3.functional.rasterization.fbo.rbo_singlesample.primitives.lines_wide

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>