`ClutterText` painting for editable single_line_mode actors like `StEntry`
is always clipped by:
`cogl_framebuffer_push_rectangle_clip (fb, 0, 0, alloc_width, alloc_height)`
So it's difficult to get the rectangle wrong. However in cases where the
target framebuffer has changed (`cogl_push_framebuffer`) such as when
updating `ClutterOffscreenEffect` we had the wrong old value of `fb`. And
so would be clipping the wrong framebuffer, effectively not clipping at all.
b8340f1355