glamor: Only use dual blending with GLSL >= 1.30

It can't be used with older GLSL. Fixes a crash when attempting to
anyway.

Closes: xorg/xserver#97
Fixes: e7308b6c "glamor: Add support for CA rendering in a single pass."
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(Cherry picked from commit 5bfca003)
3 jobs for server-1.20-branch in 4 minutes and 58 seconds
Status Name Job ID Coverage
  Docker Image
passed debian-testing #1343817

00:00:25

 
  Build And Test
passed autotools-build-and-test #1343818

00:04:32

passed meson-build-and-test #1343819

00:02:18