I don't want to ditch old Intels with GL 2.1 yet. Also now the version
reported in Context is really the highest supported version, which
optimizes extension presence checks a bit.
Apparently input events can be processed during buffer swapping. Now
asserting that callback from buffer swap is not called during input
event processing (which shouldn't, as both functions should be called in
the same thread).
NaCl and SDL2 are now hotswappable again without any unimplemented or
superfluous function parameters.
Also added missing EnumSet operators for Modifiers.
In SDL they are only for keyboard events and there is no support
for mouse modifiers like in X. Also reordered the enum in
AbstractXApplication to make it consistent.
Because with NaCl WindowContext doesn't make sense anymore, the classes
are now renamed:
* Contexts namespace -> Platform
* *WindowContext -> Application
Hopefully this is (one of) last crazy renaming runs.