opengl3: emscripten; added instance

[Horki]

Hi, @ocornut updated make file so web instance is automated; please test

[ocornut]

Hello,
Thanks for the PR.

• Perhaps instead of copying and renaming the files we should instead change the build to output everything in a folder and then just serve that folder.
• Would be good to add a reference to the serve rule in the README file.
• I don't think mkdir -p is valid under windows.
• .gitignore it missing a carriage return.
• The contents of the web rule could be folded under serve.

[Horki]

Hi @ocornut ,

• build is done inside web folder
• added small reference to README; please update if it's not clear enough
• Yes, you are right; renamed to mkdir web
• added carriage return to .gitignore
• done

[ocornut]

Thank you @Horki, squashed and merged + minor tweaks (added feaa7ea)

The Makefile are generally a bit ugly and wish the .o files were output in a folder there, but we'll tackle that more globally.