[Ext] Revive pixelui

[dusk125]

• Port and fixup pixelui
• Add Sprite Atlas and Sprite packer
• Update ui to v4 of imgui-go
• Fork and update imgui-go to latest imgui version

[dusk125]

Last I checked: The backend imgui-go was deprecated for a different project that more closely tied the backend to the project itself, rather than the imgui-go way of providing the draw data for rendering.

This basically means that either we fork and maintain imgui-go additionally or we have to potentially move to a less-supported/more difficult to integrate with library.