Fix invalid texture sizes

[parasyte]

• Makes methods fallible when they create textures.
• Breaking changes:
  • Pixels::Error is now marked #[non_exhaustive]
  • Pixels::resize_buffer() and Pixels::resize_surface() return Result<_, TextureError>.
  • Pixels::resize_buffer() no longer panics on invalid inputs.
• Correctly handle window resize in fltk example.
• TODO: tests
• Closes Properly handle dubious texture sizes #240

I haven't written any tests for this, because I still don't have a great solution to the issue raised in #80.

This does add a lot of noise to the examples. I can perhaps address some of that with Result transformation methods. The "print error and exit event loop" pattern is very common in these examples but might need more intelligent handling in a real application.

[parasyte]

It will probably be better to store the limits in the constructor instead of making this call each time. I don't know what the exact overhead is, but we only need one of many limits here, and the limit is static. This function will be called rapidly during window resizes (dragging the corner of the window with the mouse).