OpenGL 2.1 Support Broken

Compiling the example with the OpenGl 2.1 backend works, but running the example results in a blank window. The background color is set, but no UI is drawn.

How to test:

1. Replace gogl import from v3.2-core to v2.1
2. Build with go build -tags opengl2 or go build -tags "sdl2,opengl2"