gtk3 vclplug, text rendering via cairo

The LibreOffice gtk3 vclplug is currently basically rendering everything via the "svp" plugin code which renders to basebmp surfaces and then blits the result of all this onto the cairo surface belonging to the toplevel gtk3 widget

So the text is rendered with the svp freetype based text rendering and looks like this...

With some hacking I've unkinked a few places and allowed the basebmp backend to take the same stride and same same rgbx format as cairo, so we can now create a 24bit cairo surface from basebmp backing data which allows us to avoid conversions on basebmp->cairo and allows us to render onto a basebmp with cairo drawing routines, especially the text drawing ones. So with my in-gerrit-build-queue modifications it renders the same as the rest of the gtk3 desktop.