Performance and garbage optimizations to Joystick.SDL

[kimimaru4000]

Relevant issue: #6820

Summary of changes in the initial commit:

• Added a default JoystickState that's returned if the joystick isn't found to bypass allocating several zero-length arrays
• Replaced HasFlag with bitwise operators to avoid boxing
• Reduced dictionary lookups if the joystick exists by replacing ContainsKey followed by a dictionary indexer with TryGetValue

I'd like to figure out a way to let users bypass allocating the arrays involved in creating each JoystickState, as they can generate a lot of garbage over time.

[Jjagg]

You can use Array<T>.Empty instead of new T[0]. No need to cache the default instance. Otherwise, this looks good :)

[kimimaru4000]

I'm still using Protobuild to generate the project files so I'm seeing the project as .NET Framework 4.5, which doesn't have Array<T>.Empty. Should I be running the CAKE script?

[Jjagg]

Oh wow, I thought it existed way back. Nevermind then. Merging! Thanks @tdeeb

[kimimaru4000]

@Jjagg I appreciate the merge; this is definitely an improvement over before. I should have mentioned that it isn't complete yet since I wanted to discuss if we can make any further improvements when a joystick does exist - for example, an API to allow the user to supply their own arrays, or an alternative that can reduce or eliminate garbage if the user needs to.

[Jjagg]

We can discuss it further in #6820 :)