draw: fix splitting of line loops (v2)

draw: fix splitting of line loops (v2)

When the draw module splits long line loops, the sections are emitted
as line strips.  But the primitive type wasn't set correctly so each
section was being drawn as a loop, introducing extra line segments.

To fix this, we pass a new DRAW_LINE_LOOP_AS_STRIP flag to the run()
function.  The linear/elt_run() functions have to check for this flag
and set their primitive type accordingly.

No piglit regressions.  Fixes piglit's lineloop with -count 4097 or
higher.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81174

Reviewed-by: Roland Scheidegger <sroland@vmware.com>