Drop compatibility mode

ogre_media/materials/glsl150/pass_pos_color.vert

@@ -1,4 +1,9 @@
-#version 150 compatibility
+#version 150
+
+// need to use exactly these names to have OGRE bind the unpacked values:
+// https://ogrecave.github.io/ogre/api/latest/_high-level-_programs.html#Binding-vertex-attributes
+in vec4 vertex;
+in vec4 colour;
 
 // this merely passes over position and color as needed by box.geom
 
@@ -7,6 +12,6 @@ out VertexData {
 } vdata;
 
 void main() {
-    gl_Position = gl_Vertex;
-    vdata.color = gl_Color;
+    gl_Position = vertex;
+    vdata.color = colour;
 }