Implement content_fit for viewer widget

[Gigas002]

Implemented content_fit property for viewer widget to have the same behavior, image widget has. That required rework of image_size function and minor rework of draw and layout functions. Below are examples of how it's working.

On the left side is viewer widget, on the right side is image widget.

Contain

https://github.com/iced-rs/iced/assets/24297712/c270729a-eb69-437c-beab-3c82dae6aee8

Cover

https://github.com/iced-rs/iced/assets/24297712/c15a945e-a5ed-47d4-b7b1-6ca63950a7a5

Fill

https://github.com/iced-rs/iced/assets/24297712/15f6daac-39b5-45af-bdc9-cd7db0f5f897

None

https://github.com/iced-rs/iced/assets/24297712/7bd36807-ad92-4481-80e5-ef03652374eb

ScaleDown

https://github.com/iced-rs/iced/assets/24297712/5808d467-fa58-46f3-bf3c-e51cf8d9eba1

[Gigas002]

Resolved merge conflict, sorry!