Adding capacity parameter to SpriteBatch ctor

[mrhelmut]

Hey there,

I'd like to propose the addition of a parameter to the SpriteBatch constructor to have a control over its initial batch items capacity. The intention is to give a finer control of the memory allocations and garbage generation to developers (e.g. increasing the batch size right away to avoid any runtime array resizing).

This PR shouldn't break anything and should remain backward compatible.

@Jjagg @cra0zy @tomspilman

[Jjagg]

I like this. Thanks @mrhelmut, merging!

[harry-cpp]

This is an API change, we shouldn't merge stuff like this without @tomspilman or others consent.

[Jjagg]

It's not breaking, straightforward and very minor so I figured it was fine, but yeah.

[tomspilman]

This is ok... Very easy thing to add that shouldn't cause any troubles on console.

[harry-cpp]

[harry-cpp]

I sure do hope somebody gets the reference XD