Thank you!

I also read that in previous versions imgui used square instead of a checkmark, maybe I could try to find older version.
What about image filtering, what do you think? Will that also require building own dll?

Edit: I think I found old version of RenderCheckMark function (if someone interested)
Nope, it doesn't work... So I left the checkmark as it is, tried to draw X but failed

And for disabling linear filtering, indeed, changing D3DTEXF_LINEAR to D3DTEXF_NONE in imgui_impl_dx9.cpp works.