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: https://gitlab.freedesktop.org/xorg/xserver/issues/97
Fixes: e7308b6c77 "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 5bfca0038e)

glamor/glamor.c

@@ -624,6 +624,7 @@ glamor_init(ScreenPtr screen, unsigned int flags)
         epoxy_gl_version() >= 30 ||
         epoxy_has_gl_extension("GL_NV_pack_subimage");
     glamor_priv->has_dual_blend =
+        glamor_priv->glsl_version >= 130 &&
         epoxy_has_gl_extension("GL_ARB_blend_func_extended");
 
     glamor_priv->can_copyplane = (gl_version >= 30);