Better canvas cleanup, avoids segfaults on some systems with glfw+wgpu

[almarklein]

Ref pygfx/pygfx#642

• Replace calls to ._rc_close() with .close()`
• In close() do additional cleanup for the canvas, because these can hold onto (wgpu) resources:
  • The draw function.
  • The canvas context.
  • The event handlers (sanity check; likely already happened).
• Delay glfw termination so it happens after the wgpu surface is deleted.