Upgrade ImGui to version 1.75

[JoelLinn]

In the meantime a new ImGui version was released. This is a rebase of #27, everything in there still holds true.

Additionally these functions got removed (these plus new upstream gl(fw) impls are the only changes):

• show_test_window()
• set_next_window_content_width()
• get_items_line_height_with_spacing()

[JoelLinn]

Do you have any concerns about the changes? We can discuss them and modify this PR if so.

[podgorskiy]

All looks great! Thank you! Sorry for not merging it for a long time, I was very busy