Allow cimgui.h to be included in a plain C project
Allow cimgui.h to be included in a plain C project

Add `struct` keywords where required.  The messy part is the ImFont::Glyph inner
struct, which needs to match the implementation in fontAtlas.cpp when building
the library, but the namespace syntax is invalid C, so we have to use a macro to
satisfy both use cases.

Extrawurst/cimgui,Extrawurst/cimgui,sonoro1234/cimgui,sonoro1234/cimgui