GLSL: XFB: more support for built-in block redeclarations with XFB.

- correct inheritence (or not) of the right XFB buffer
- compute implicit stride (fixes #1212)
- semantic check block-member redeclarations
- inherit stride from a member

Test/spv.xfb2.vert

@@ -0,0 +1,18 @@
+#version 450
+
+layout (location = 0) in vec4 position;
+layout (binding = 5) uniform ComponentsBlock
+{
+    vec4 c1;
+    vec2 c2;
+} components;
+
+layout (xfb_buffer = 3, xfb_offset = 16) out gl_PerVertex
+{
+    vec4 gl_Position;
+};
+
+void main()
+{
+    gl_Position = position + components.c1 + vec4(components.c2, 0.0, 0.0);
+}