ParseHelper: assign xfb_offset for struct members too

If the out variable is a struct type, with a xfb_offset explicitly
assigned, the members need to get their xfb_offset assigned. This is
specially relevant, as we cannot use layout qualifiers on struct
members.

Test/spv.xfbOffsetOnStructMembersAssignment.vert

@@ -0,0 +1,23 @@
+#version 450
+
+layout(xfb_buffer=2) out;
+
+struct S {
+   float x1_out;
+   float x2_out;
+};
+
+layout(location=0, xfb_offset = 16) out S s1;
+
+layout(location=5, xfb_buffer=1, xfb_offset=8) out struct S2 {
+   float y1_out;
+   vec4 y2_out;
+}s2;
+
+void main() {
+   s1.x1_out = 5.0;
+   s1.x2_out = 6.0;
+   s2.y1_out = 7.0;
+   s2.y2_out = vec4(1.0, 0.0, 0.0, 1.0);
+   gl_Position = vec4(0.0);
+}