Do not draw a window before its client has painted anything

Reported as "before a window spawns, the border corners look funny".

A toplevel is placed, sized and decorated the moment its role is created,
which is well before the client draws. srdwm was rendering it from that
moment, so what appeared first was an empty frame: border, titlebar and
shadow standing around bare desktop, at the guessed 800x600 placeholder
size, with nothing inside. When the real buffer arrived the frame snapped
to the real size.

Measured in a nested session, capturing a cold terminal's spawn with grim:
four consecutive captured frames spanning 540ms showed a complete red
border with zero client content inside it, at 642px outer height, which
then settled at 610 - a jump of exactly one TITLEBAR_HEIGHT. After this
change the same capture has no such frame at all: every frame that shows a
border shows content in it, and the height does not change afterward.

Two parts:

- Nothing is drawn for a window that has never committed a buffer. All
  five paths that draw a frame agree on this - both udev render loops
  (Pixman and GPU), the winit render loop, and both screencopy paths, so a
  screenshot cannot show a frame the screen does not.
- The open-slide starts at the first commit that carries a buffer rather
  than at role creation. A cold terminal took ~800ms to paint, long enough
  for the whole tween to finish against the empty frame, so the window
  simply appeared, already at rest, with no animation at all. It now
  animates where it can actually be seen.

The answer latches once true (windows_shown_once), so a window that has
legitimately shown something is never hidden again by this however its
buffer state changes. A window that cannot be resolved to a surface counts
as drawable, deliberately: this hides a window only on positive evidence
that it has never drawn, so nothing whose surface plumbing works
differently - an XWayland window - can be hidden by a lookup that did
not apply to it.

Same shape, and the same reason, as sync_layer_visibility's own has_buffer
branch, which layer surfaces have had all along.

533 tests pass, clippy clean.

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