cimgui: handle upstream breaking changes

utils/vector: `Slice()` method is broken

TextureData.Pixels() and PixelsAt() lead to possible misuse of unsafe.Pointer

Missing ImTextureID_Invalid

Problem with TextureData.Updates()

How to use cimgui-go in a seprate window when my game using ebitengine?

Ebitenbackend: text manipulation keyboard commands not working

Are there any projects that use cimgui-go in production?

Color Text Edit: does not respect Alt Gr + key characters

Missing GetContentRegionAvail