egl: Set the draw buffer to GL_BACK_LEFT

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 9c4c4eaaa121d784f591a591b95264647eaece96 for a longer
discussion).

Fixes #4402