Improve sixel support

[mfontanini]

This stops using viuer to render images using sixel and instead does something very similar but within this codebase. The reason for this is when testing sixel support in wezterm I would get tiny images, which is because the font size is hardcoded in viuer. Here instead we pull it from the config file so you can configure it to be whatever works for your environment.

This also allows forcing the use of sixel. Because viuer always checks if a mode is supported before using it, there's no way to override that decision and say "use sixel!". With this change, if you pass in --image-protocol sixel it will use it (which means if you don't support it it will break).