The move away from `nullptr` to NoCreate for constructing an application
without creating OpenGL context was done quite some time ago for
windowless application, but for some weird reason it was never done for
windowed apps. Now made this consistent.
The old `nullptr`-based constructor is still present, but marked as
deprecated and due to be removed in some future release.
The parameterless Platform::Context::Context() constructor and
Platform::Context::tryCreate() function are deprecated in favor
functions that take argc/argv pair. The Context class now accepts
arguments starting with --magnum-* prefix. Currently there are none
except for the implicit --magnum-help, but that will change with the
following commits.
In some cases the GL context creation might success without error, but
the created version is one that we don't want (e.g. software GDI
rasterizer on Windows). Previously the Context class constructor just
exited the application and it was impossible to react on that from the
application side (for example reducing some context feature
requirements).
Now there is Platform::Context::tryCreate(), which returns either
created instance or `nullptr` if the instance creation failed with some
error. That is now used in all Platform::*Application implementations.