Uncomment the Pause/Resume functions

[mkunkacnc]

So that they can be called where implemented. Dummy functions already exist for the OSes where they are not implemented.

[icculus]

It seems like important functionality to just maybe do nothing on some platforms, though...

[sezero]

I hate those commits with a message like "Update some_source.c" which doesn't say anything about what it does in the commit history. Can you please squash tour commits, and change the commit message to SDL_native_midi.h: Uncomment the Pause/Resume functions ?

[icculus]

I think for Haiku, we probably need to change virtual void Run() to:

• Have a real time in startTime instead of B_NOW
• If paused, SDL_Delay() a little and try again (or get fancy and have a Condition Variable so we sleep until stopped/unpaused).
• When resumed, update startTime with the difference.
• Only call SprayEvent when not paused.

But I don't have a convenient way to test this, so for now, let's leave it unimplemented, merge this PR, and wait for bug reports, which may never arrive.