Take sf::Sprite's transform into account in ImGui::Image overload for sf::Sprite

[eliasdaler]

Right now sprite scale/rotation, etc. are ignored when drawing with ImGui::Image. However, this can be fixed by following this: ocornut/imgui#1982

(Will need to compute where vertices of sf::Sprite are using its transform)

Related to #86

[eliasdaler]

I feel like ignoring rotation/scale etc. of sf::Sprite is a right thing to do, since trying to handle all transforms inside this overload introduces a lot of complexity into ImGui-SFML.