material: don't declare backend vtables in headers

This fixes the material backends to declare their constant vtable in the
c file with a corresponding extern declaration in the header. This
should fix complaints about duplicate symbols seen on OSX.

cogl/cogl-material-arbfp-private.h

@@ -30,7 +30,7 @@
 
 #include "cogl-material-private.h"
 
-const CoglMaterialBackend _cogl_material_arbfp_backend;
+extern const CoglMaterialBackend _cogl_material_arbfp_backend;
 
 #endif /* __COGL_MATERIAL_ARBFP_PRIVATE_H */
 

cogl/cogl-material-arbfp.c

@@ -108,6 +108,8 @@ typedef struct _CoglMaterialBackendARBfpPrivate
   int next_constant_id;
 } CoglMaterialBackendARBfpPrivate;
 
+const CoglMaterialBackend _cogl_material_arbfp_backend;
+
 static int
 _cogl_material_backend_arbfp_get_max_texture_units (void)
 {

cogl/cogl-material-fixed-private.h

@@ -30,7 +30,7 @@
 
 #include "cogl-material-private.h"
 
-const CoglMaterialBackend _cogl_material_fixed_backend;
+extern const CoglMaterialBackend _cogl_material_fixed_backend;
 
 #endif /* __COGL_MATERIAL_FIXED_PRIVATE_H */
 

cogl/cogl-material-fixed.c

@@ -54,6 +54,8 @@
 #include "../gles/cogl-gles2-wrapper.h"
 #endif
 
+const CoglMaterialBackend _cogl_material_fixed_backend;
+
 static int
 _cogl_material_backend_fixed_get_max_texture_units (void)
 {

cogl/cogl-material-glsl-private.h

@@ -30,7 +30,7 @@
 
 #include "cogl-material-private.h"
 
-const CoglMaterialBackend _cogl_material_glsl_backend;
+extern const CoglMaterialBackend _cogl_material_glsl_backend;
 
 #endif /* __COGL_MATERIAL_GLSL_PRIVATE_H */
 

cogl/cogl-material-glsl.c

@@ -54,6 +54,8 @@
 #include "../gles/cogl-gles2-wrapper.h"
 #endif
 
+const CoglMaterialBackend _cogl_material_glsl_backend;
+
 static int
 _cogl_material_backend_glsl_get_max_texture_units (void)
 {