cogl-vertex-buffer: Add support for unsigned int indices

cogl-vertex-buffer: Add support for unsigned int indices

This adds a COGL_INDICES_TYPE_UNSIGNED_INT enum value so that unsigned
ints can be used with cogl_vertex_buffer_indices_new.  Unsigned ints
are not supported in core on GLES so a feature flag has also been
added to advertise this. GLES only sets the feature if the
GL_OES_element_index_uint extension is available. It is an error to
call indices_new() with unsigned ints unless the feature is
advertised.

http://bugzilla.openedhand.com/show_bug.cgi?id=1998