This makes the code consistent with other: Mac, Unx, Windows
backends, and allows us to cache an OpenGL back-buffer on the
SalGraphics - as per Mac.

The lifecycle of a SalFrame and a SalGraphics should be
consistent for real OS graphics now.

Change-Id: I11fea2ce1c1386b1c6b0161a718e5c909c81612c
Reviewed-on: https://gerrit.libreoffice.org/20283Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>