Implement composition engine with damage tracking

[pbalduino]

Summary

Implement the composition engine that efficiently renders all windows to the screen using damage tracking and dirty rectangle optimization.

Goals

• Render windows in Z-order
• Damage tracking (only redraw changed areas)
• Double buffering
• Vsync synchronization
• Performance optimization

Implementation

Damage Tracking

```c struct damage_rect { int x, y, width, height; struct damage_rect *next; };

void compositor_damage(struct compositor *comp, int x, int y, int w, int h) { struct damage_rect *rect = malloc(sizeof(*rect)); rect->x = x; rect->y = y; rect->width = w; rect->height = h; // Add to damage list comp->damage_dirty = 1; } ```

Composition

```c void compositor_render(struct compositor *comp) { cairo_t *cr = cairo_create(comp->screen);

// Clear damaged areas
cairo_set_source_rgb(cr, 0.2, 0.2, 0.2);
cairo_paint(cr);

// Composite each window
for (struct window *w = comp->windows; w; w = w->next) {
    if (!w->visible) continue;

    cairo_set_source_surface(cr, w->surface, w->x, w->y);
    cairo_rectangle(cr, w->x, w->y, w->width, w->height);
    cairo_fill(cr);
}

cairo_destroy(cr);

} ```

Timeline

Total: 2-3 weeks

Definition of Done

• [ ] Windows composite correctly
• [ ] Damage tracking works
• [ ] Only damaged areas redrawn
• [ ] Smooth 60fps
• [ ] No tearing

Dependencies

• #396: Cairo
• #403: Compositor core

See docs/road/road_to_gui.md for complete roadmap.