Replace struct gl_texure_object's Dimension w/ Target field.
Added _EnabledUnits to struct gl_texture_attrib - the _ReallyEnabled
  field is obsolete, but still present for now.  This effectively
  removes the 8-texture units limit, 32 units now possible, but unlikely!
New TEXTURE_1D/2D/3D/CUBE/RECT_BIT tokens for unit->_ReallyEnabled field.
Updated device drivers to use ctx->Texture._EnabledUnits.