clutter-stage-cogl: Fix uninitialized variable `use_clipped_redraw`

I noticed my system would fall back to the slow unclipped (and uncullable) paint path whenever a window touched the left edge of the screen. Turns out that was a red herring. Just that use_clipped_redraw was uninitialized so clipping/culling was used randomly.

So the compiler failed to notice use_clipped_redraw was uninitialized. Weirdly, as soon as you fix that it starts complaining that buffer_age might be uninitialized, which appears to be wrong. So we initialize that too, to shut up the compiler warnings/errors.