slideshow mbHasTFPVisual is always false since

commit eccaf91e
Date:   Mon May 19 19:21:29 2014 +0200

    use VCL's OpenGLContext for 3D transitions

    Change-Id: I510518461eb8bc9669d0de2679c34c473f66b175

and GLWindow fbc has always been null since that incarnation of opengl started,
so even if mbHasTFPVisual was true it would crash.

OpenGLHelper::GetPixmapFBConfig is *almost* the same as the old removed code
for setting fbc, but if I use that then for me the transitions still don't work
at all.

Examining further shows GetPixmapFBConfig lacks the test for
GLX_BIND_TO_MIPMAP_TEXTURE_EXT that the old code had.

If I add than, then it "works", but examining *that*, reveals it only works
because GLX_BIND_TO_MIPMAP_TEXTURE_EXT is unsupported on my rig, so the
GLX_EXT_texture_from_pixmap code still doesn't get executed.

I apparently can't test the original working configuration, and I'm not
particularly interested in getting X working and I just wanted to make sure I
didn't break that case, so...

this removes the uncallable since 2014 code entirely rather than try to
fix it.

I suspect this may leave the cairo-canvas CanvasBitmap::getFastPropertyValue
"1" branch now also dead

Change-Id: I6727576056533fa54a4f82378954fb53891f5873