Backends: Win32: dynamically load XInput library

[Demonese]

See: #3645
Related Pull Request:#3248

[ocornut]

Thank you @Demonese.
This looks good but I'm not going to merge asap as I'm aiming to tag 1.80 soon and don't want to introduce unknown there, but this should be good for 1.81.

I noticed your fixed the batch file, thanks! This was indeed overlooked by b1a18d8 and I've now fixed it with 3346544 (your PR will conflict because of it, but you can remove the modification to the .bat file and rebase over master + force push if you want, otherwise I'll do it when merging).

I think if you keep XINPUT_STATE xinput_state; on the stack as this was the patch would not need to add the extra indentation in ImGui_ImplWin32_UpdateGamepads().

Thanks again!

[Demonese]

@ocornut Ok, it's all right now. I will keep this PR up to date because my project is using this improvement.

[Demonese]

😋I refer to GLFW's implementation and added searches for XInput1_1.dll and XInput1_2.dll. I also added some cleanup code (line 151-155, maybe you forgot them? @ocornut ):

[ocornut]

Now merged this, thank you @Demonese!

Your commit 633d103 + various minor amends f139846 (using LoadLibraryA as used elsewhere in the file, moved xinput state back to the stack is simpled and shorter, fixed spacing and minor coding style fixes).

Thanks a lot!