gsk/gl: use GdkArrayImpl for Clip tracking

gsk/gl: use GdkArrayImpl for Clip tracking

We can end up spending a lot of time in g_array_maybe_expand() through the
use of g_array_set_size() for clip tracking. That is somewhat due to the
simple nature of GArray being size-dynamic. Instead, we can use
GdkArrayImpl and let the compiler do what it does best to elide some
work and hoist other work into the calling function.

This also fixes a potential UAF in gsk_gl_render_job_push_contained_clip().