vulkan: Put the vertex buffer into the render object

vulkan: Put the vertex buffer into the render object

Renderpasses get recreated every frame, but we keep render objects
around. So if we keep the vertex buffer in the render object, we can
also keep it around and just reuse it.

Also, we only need one buffer for all the render passes, which is
another bonus.

The initial buffer size is chosen at 128kB. Maximized Nautilus,
gnome-text-editor with an open file and widget-factory take ~100kB when
doing a full redraw. Other apps are between 30-50kB usually.

So I chose a value that is not too big, but catches ~90% of cases.