[AD] primitives thickness=1

[Peter Wang <novalazy@xxxxxxxxxx>]

Should the high-level primitives routines draw anything with thickness
<= 1.0 (or even < 2.0 ?) using LINE_STRIPs instead of TRIANGLE_STRIPs?
The [test hl thick-1] shows many skipped pixels, whereas I expect it to
look like [test hl thick-0].

Peter