glamor: Fix dashed line rendering.

We were binding the screen pixmap as the dash and sampling its alpha,
which is usually just 1.0 (no dashing at all).

Please cherry-pick this to active stable branches.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit fe0b2974)