Similar change to the windowed apps, also resetting it when core context
creation fails or when the workaround is applied. This change is not
done for CGL, iOS and Windows EGL apps, as these are either GLES-only
or, in case of macOS, such flag is not available.
There's much more to work around / fix, but this is a start. First we
need to create the context with a pbuffer, otherwise eglMakeCurrent()
crashes deep inside. Second, it doesn't treat EGL_CONTEXT_FLAGS_KHR as a
bitfield, so it blows up when encountering a combination of zero flags.
In that case we're simply not sending the flags there. This would also
blow up when there's more than one flag passed, but there's just one
flag for debug context at the moment, so shouldn't be a problem.
Each Windowless*Application has now a companion Windowless*Context that
manages just the GL context creation and nothing else, with the ability
to just create the context and not make it current, so it doesn't affect
current thread state and can be moved to another thread and make current
there.
Other things that were done:
* Using `NoCreateT` instead of `nullptr` for creating the application
without creating GL context.
* Properly handling failed creation of Magnum context instance -- if it
errors out, also the GL context is destroyed to make it possible to
create the context with a different configuration.
* Reworked AMD and NVidia binary driver workaround, where core context
created with specific version doesn't automatically choose the newest
available (creating compatibility context on the other hand causes
the version to get stuck on 2.1 on Mesa and OSX).
* Added the above workaround also for WindowlessWglApplication to avoid
driver issues in the future.
* Reworked WindowlessWglApplication to not be so crazily entangled. It
was a misunderstanding on my side about how WINAPI works. Much
simpler now (and I hope still working :D).
Uses just EGL and no other platform toolkit, meaning that the same code
can be used on all platforms if the drivers support it. Should be
working for OpenGL ES for most drivers, however desktop OpenGL is
supported only on NVidia since version 355.
Vladimír Vondruš