SPV: Implement new extensions GL_KHX_device_group and GL_KHX_multiview.

These correspond to SPV_KHR_device_group and SPV_KHR_multiview.
Also, bring tests up to date with Khronos internals, and some misc. related changes.

Test/spv.multiView.frag

@@ -0,0 +1,9 @@
+#version 450
+
+#extension GL_KHX_multiview : enable
+
+out vec4 color;
+
+void main() {
+    color = vec4(gl_ViewIndexKHR, 0, 0, 0);
+}