FixedTextureAtlasManager should use ref-counted textures properly.
Also - dispose embedded textures early in VCL shutdown while we have
a valid OpenGLContext.
Also - dispose the native widget control cache earlier too.

Change-Id: Id3f7a1c3b331496616f36cbf02f83737505278a5
Reviewed-on: https://gerrit.libreoffice.org/21148Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>