VESA Unit - Add VESA support to Gr's capabilities


Include the VESA unit into your USES list, and all VESA modes provided
by GR will be supported. They can be initialized via InitGraphics, and
the following of our graphics packages will support them:

MyMouse, Version 1.60+

  Support for all 16-color modes up to 800x600x16, and
  support for all 256-color modes up to 1024x768x256.
