Semantics: Geometry stage can support max_vertices = 0.

To do this, more generally use a named -1 as a not set value.

Test/max_vertices_0.geom

@@ -0,0 +1,12 @@
+#version 330
+
+layout(points) in;
+layout(triangle_strip, max_vertices = 0) out;
+in highp vec4 v_geom_FragColor[];
+out highp vec4 v_frag_FragColor;
+
+void main (void)
+{
+    EndPrimitive();
+    EndPrimitive();
+}