Fix BreakneckLock

[ZingBallyhoo]

Summary of the PR

Fixes the implementation of BreakneckLock (only used in SDL2 event code).
Found this issue when trying to figure out why the SDL2 platform implementation had a low update rate for inputs. (first frame locking fails but actually acquires, second frame "succeeds" to acquire the already acquired lock)

Consider the following code: (similar to platform impl)

var l = new BreakneckLock();
var taken = false;
Console.Out.WriteLine($"IsAcquired before: {l.IsAcquired}");
l.TryEnter(ref taken);
Console.Out.WriteLine($"---- TryEnter ----");
Console.Out.WriteLine($"taken after: {taken}");
Console.Out.WriteLine($"IsAcquired after: {l.IsAcquired}");
taken = false;
l.TryEnter(ref taken);
Console.Out.WriteLine($"---- TryEnter ----");
Console.Out.WriteLine($"taken after 2: {taken}");
Console.Out.WriteLine($"IsAcquired after 2: {l.IsAcquired}");
taken = false;
l.TryEnter(ref taken);
Console.Out.WriteLine($"---- TryEnter ----");
Console.Out.WriteLine($"taken after 3: {taken}");
Console.Out.WriteLine($"IsAcquired after 3: {l.IsAcquired}");

Current implementation:

IsAcquired before: False
---- TryEnter ----    
taken after: False    
IsAcquired after: True
---- TryEnter ----
taken after 2: True
IsAcquired after 2: True
---- TryEnter ----
taken after 3: True
IsAcquired after 3: True

Fixed implementation:

IsAcquired before: False
---- TryEnter ----
taken after: True
IsAcquired after: True
---- TryEnter ----
taken after 2: False
IsAcquired after 2: True
---- TryEnter ----
taken after 3: False
IsAcquired after 3: True

Further Comments

The bug was added in this commit john-h-k/SpinLockSlim@53de583

[Perksey]

cc @john-h-k as the original author

[Perksey]

Looks good to me, thanks for your contribution!