Backends: OpenGL3: Support older 2.x series of glbinding as loader for OpenGL3 (#3061)

This removes the unversioned definition IMGUI_IMPL_OPENGL_LOADER_GLBINDING in favor of two versioned ones to choose explicitly.
References: #2870, 5e2329b98e6562c9e7883e92dcf878e969d81878
6 files changed