Add support for VK_EXT_fragment_shader_barycentric

Test/spv.fragmentShaderBarycentric3.frag

@@ -0,0 +1,15 @@
+#version 450
+#extension GL_EXT_fragment_shader_barycentric : require
+
+layout(location = 0) pervertexEXT in vertices {
+    float attrib;
+    } v[];   
+      
+layout(location = 1) out float value;
+      
+void main () {
+    value = (gl_BaryCoordEXT.x * v[0].attrib +
+             gl_BaryCoordEXT.y * v[1].attrib +
+             gl_BaryCoordEXT.z * v[2].attrib);
+
+}