GWN: Element Buffer: Refactor / Optimisation.

 - Upload the data to the GPU directly when creating the element buffer in
   GWN_indexbuf_build_in_place().

 - Convert data in place when squeezing the indices and removing the need
   for another allocation.

 - GWN_indexbuf_build_in_place() can be used with already used element
   buffers and reupload their data without changing vbo id (keeping vaos
   up to date).