fix multi-window issues + add accessors to configure various rendering features (textures, shadows, anti-aliasing)

[sebcrozet]

New Features

• OffscreenSurface renders with MSAA: OffscreenSurface::set_samples / samples, or
  CanvasSetup::samples through with_setup. OffscreenSurface::new stays single-sampled.
  Offscreen frames used to ignore the canvas sample count and always render one sample.
• OffscreenSurface::set_shadow_softness / shadow_softness and set_shadows_enabled /
  shadows_enabled, mirroring the Window methods.
• ShadowMapper::set_atlas_layers (Window / OffscreenSurface::set_shadow_atlas_layers)
  allocates fewer than the 16 shadow atlas layers, which makes a finer shadow resolution
  affordable when a scene only lights with one or two directional lights.
• TextureManager::set_anisotropy: anisotropic filtering (up to 16x) for textures loaded with
  mipmaps and linear filtering, which keeps floors and other grazing-angle textures sharp.
• Window and OffscreenSurface expose the directional cascade layout that only the
  ShadowMapper had: set_shadow_distance / shadow_distance and
  set_first_cascade_far_bound / first_cascade_far_bound.
• Added the offscreen_msaa example: an antialiased, hard-shadowed offscreen render.

Bug Fixes

• Creating a second Window or OffscreenSurface on a thread that already has one no longer replaces the global mesh, texture and material managers. Objects created before the new surface kept the previous default material, which the per-frame begin_frame / flush no longer reached, so its per-object uniform buffer grew until wgpu rejected the dynamic offsets (a validation error after 256 draws). The managers are now created once per context and reused until the last window closes.
• Added the offscreen_shared_managers example exercising a scene built before its offscreen surface, with shaded renders interleaved with depth and segmentation read-backs.