blur-effect: Floor downscaled frambuffer sizes

blur-effect: Floor downscaled frambuffer sizes

Floor the downscaled size of the new framebuffer to make sure we don't
initialize a framebuffer with a floating point value that might be
interpreted wrong by `cogl_texture_2d_new_with_size` and end up with a
slightly wrong aspect ratio of the framebuffer.

This fixes situations where the widths or heights of downscaled
framebuffers sometimes miss some pixels at the border.

While at it, remove the `downscale_factor` argument from
`setup_projection_matrix` since that function doesn't need the initial
size of the actor anyway.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/991