wayland: Add opt-in border inset window properties

[svncibrahim]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

A client that draws its own decorations (shadows, an invisible resize border) has no way on Wayland to declare that the window geometry is the surface minus those insets, so snap, tile and maximize align to the surface edges instead of the visible content.

This adds four opt-in window properties:

  SDL_PROP_WINDOW_WAYLAND_BORDER_INSET_LEFT_NUMBER
  SDL_PROP_WINDOW_WAYLAND_BORDER_INSET_TOP_NUMBER
  SDL_PROP_WINDOW_WAYLAND_BORDER_INSET_RIGHT_NUMBER
  SDL_PROP_WINDOW_WAYLAND_BORDER_INSET_BOTTOM_NUMBER

When set, SDL sets the xdg window geometry to the surface minus the insets and translates configure sizes, size limits and popup positioning accordingly. All-zero insets (the default) leave the geometry call and the protocol traffic identical to today.

The insets apply only to xdg-toplevel windows and are skipped while maximized or fullscreen, where an exact size is required. The configure-size dedup is now state-aware, so a maximize or fullscreen toggle at unchanged dimensions is re-adopted instead of skipped.

Existing Issue(s)

• Fixes Wayland: let a client declare border insets #16058

[Kontrabant]

A few initial comments.

Just to let you know, I'm going to be traveling later this week, and won't be back until near the end of the month, so if I'm slow to respond, that's why. A final merge will have to wait until I'm back as well.