gdkglcontext-wayland: Fallback to GLES 2.0 after GL 3.0 failed
This fixes the problem that GDK unable to create a GL context on Wayland when OpenGL has unsupported version (<3.0), but OpenGL ES 2.0 is supported. This patch implements a fallback, so users don't need to specify GDK_GL=gles
explicitly on those systems.
It can be reproduced by running the "OpenGL Area" example from gtk3-demo with MESA_GL_VERSION_OVERRIDE=2.1
on Wayland.
Fixes #2619
Edited by Balló György