Because ClutterText has a somewhat convoluted drawing routine,
replacing cogl_rectangle() here isn't as straightfoward as the
effects were.

A new CoglPipeline is now part of the ClutterText struct, and
is used to set the color of the  background or the selection.

Another change is paint_selection() now receives a framebuffer
to draw into. The check for NULL framebuffer does not make
sense here, since there is always a draw framebuffer set
when in the drawing function. Because of that, the check is
now gone.