fix: example's BGFX initialization

[funatsufumiya]

I wondered why example hello world is not working, and I found solution. I confirmed bgfx initialization method is changed recently (I don't know when it changed).

[funatsufumiya]

(P.S.) solved on 256f2ea

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Sorry, I confirmed this version makes errors below on Windows environment. I'll check what's happening here.

SDL2 version: 2.0.5 - Subsystem: SysWM_Windows
Traceback (most recent call last)
C:\Users\fu\lab\bgfx.nim\examples\00-HelloWorld\main.nim(35) main
C:\Users\fu\lab\bgfx.nim\examples\graphics.nim(142) init
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

I also checked the version before PR, shows error below. So I think initialization method itself is not wrong.

SDL2 version: 2.0.5 - Subsystem: SysWM_Windows
Error initializng BGFX.