egl: Set the draw buffer to GL_BACK_LEFT
Instead of using GL_BACK, use GL_BACK_LEFT, because the spec demands this (many drivers don't).
Also move the call from the GDK backends into the GLContext code, as this is a generic EGL issue (nvidia being the main driver in need of this call, see 9c4c4eaa for a longer discussion).
Fixes #4402 (closed)