Improve code style, add missing const qualifiers

[halx99]

This will also fix compile error when invoke FindGlyph of a const instance of class ImFont

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[ocornut]

Hello,
As explained #4938 this is mostly undesirable: making those functions use const increase the contract surface that ImGui has to honor.
Very concretely I am right now working on changes that requires those to not be const anyway.
Closing this.