The background image needs to be scaled to fit current texture size.
Previously this was done by gdk_pixbuf_scale_simple but that's been
removed.
Create a texture from the background pixbuf with correct dimensions and
use interpolation shader to scale it to the right size. Interpolation
fragment shader doesn't have too much sense if all the textures don't
have the same size so this seemed the most natural place to do the
scaling. It could probably be done with some custom texture mapping
outside the shader but it involved more code.
Fixes bug #599883.
