Add demo for glfw + opengl4 without any gl extensions

[aichingm]

The current glfw + opengl4 demo makes use of the ARB_bindless_texture extension which is not supported on all platforms in all drivers. On linux for example this extension is only supported in newer Nvidia and AMD drivers (not intel, apple, arm, ...) Source.

This PR renames the existing glfw_opengl4 demo to glfw_opengl4_ext and adds a newglfw_opengl4_core demo which does not make use of any opengl extensions.

PS: I'm sorry to sneak a second commit into this PR: it adds some not ignored demo binaries to .gitignore, mea culpa.