window: Add meta_window_get_client_area_rect
window-actor: Don't pass around client_area

To solve a performance regression, the three update_*_region
calls are going to be split up, so just call get_client_area_rect
in the methods itself.
window-actor: Kill off another use of MetaFrameBorders
window: Update the shape, input, and opaque regions immediately

... and individually. It turns out that updating the opaque region
was causing the shape region to be updated, which was causing a new
shape mask to be generated and uploaded to the GPU. Considering
GTK+ regenerates the opaque region on pretty much any focus change,
this is not good.
window-actor: Complete the removal of update_shape

I accidentally stashed the rest of the stuff and forgot to
commit it... oops.