Support for FP render functions

[xsbee]

SDL2, as of now, has documented functions for rendering at floating-point coordinates. Integer coordinates can be quite troublesome at times and often is not desirable.

It surfaced in a commit of 2018 and was undocumented for a long time according to this. However the changelogs apprarently seem to record it. As none usually talks about it, it slipped through notice easily.

It is so useful that, in fact I had to use unsafe FFI in my program to use this interface.

https://github.com/xsbee/rvsp/blob/d1f8ea3091fadd45e8e3b8237db842545c51fdb2/src/main.rs#L159-L162

So kindly consider adding this inteface. Thank you.

[Cobrand]

Sure, PRs are welcome!