With this flag set (which is done implicitly for all windowless apps
and, conversely, not done for all windowed apps), the default
framebuffer state isn't touched in any way, which should avoid potential
race conditions with default framebuffer on another thread.
This removes one unnecessary allocation from each application startup.
In some cases of the windowless apps the Platform::GLContext could be
put directly into the class, in other cases it had to be wrapped in an
Optional because we need delayed construction and/or earlier
destruction.
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).
Vladimír Vondruš