Use own embedded OpenGL loader

[moritz-h]

Replace the OpenGL loading defines by an own minimal embedded OpenGL loader. This is the same as upstream imgui is doing now, see ocornut/imgui#4445.

The changes to tex_inspect_opengl.cpp are taken from the upstream imgui repo. The loader is generated with the command:

./gl3w_gen.py --output ../imgui_tex_inspect/backends/tex_inspect_opengl_loader.h --ref ../imgui_tex_inspect/backends/tex_inspect_opengl.cpp ./extra_symbols.txt --symbolprefix imtexin_

There is a patch needed for gl3w_gen.py to allow overwriting the symbol prefix to avoid conflicts with the loader used by imgui itself, a PR with that feature is here: dearimgui/gl3w_stripped#1