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)