buffer: explicitly relate buffers to a context

All CoglBuffer constructors now take an explicit CoglContext
constructor. This is part of the on going effort to adapt to Cogl API so
it no longer depends on a global, default context.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

examples/cogl-hello.c

@@ -29,7 +29,7 @@ main (int argc, char **argv)
     cogl_onscreen_show (onscreen);
     fb = COGL_FRAMEBUFFER (onscreen);
 
-    triangle = cogl_primitive_new_p2c4 (COGL_VERTICES_MODE_TRIANGLES,
+    triangle = cogl_primitive_new_p2c4 (ctx, COGL_VERTICES_MODE_TRIANGLES,
                                         3, triangle_vertices);
 
     pipeline = cogl_pipeline_new ();