x11-servers/xorg-server: Fix dashed/dotted line rendering with GLAMOR.

When drawing dashed lines with GLAMOR, they were drawn partially or as
a regular (solid) line, depending on the OpenGL backend. It behaved
so because screen pixmap was bound as the dash and sampling its alpha,
which is usually just 1.0 (no dashing at all).

Upstream bug: https://bugs.freedesktop.org/show_bug.cgi?id=99708
Fixed by: Eric Anholt (it's a pity that anholt@ no longer works

		on FreeBSD)

Silence from: x11@