xrRender: static geometry buffers code unification (#489)

Слиты xottab-duty запросил слияние github/fork/vTurbine/renderer/staging_buffers в xd_dev

Created by: vTurbine

  • Added staging buffer wrapper for static geometry;
  • Common buffer interface for DX9/DX10/DX11/VK
  • Backend top file simplified and made common

Please check if it works fine on Linux as well: I don't have OpenGL 4.x on my VM