Glade should have a process for unloading screens

[profexorgeek]

Screens currently have no unloading process to dispose of textures, sprites, layers, etc.

• Glade should be updated so that screens have an unloading process.
• The base Screen class should have a virtual unload method
• Sprites should be recursively unloaded and destroyed on unload
• As part of unloading the LayerManager should clear any layers

[profexorgeek]

@KallDrexx I know you changed the unloading process recently. Have you completed any/all of these requirements

[KallDrexx]

Screen unloading is done by TransitionToScreen(), which will call Dispose() on the screen if it implements IDisposable: https://github.com/profexorgeek/Glade2d/blob/main/Glade2d/Game.cs#L107

Not sure if there's more to it that's needed.