Use KHRONOS_APIENTRY to fix Win32 crash

[paradust7]

Proposed solution to Windows 32-bit build crash.

The crash is caused by a calling convention mismatch. The default calling convention is cdecl, but the OpenGL functions returned by getProcAddress() are stdcall. This causes stack corruption unless function pointers are annotated with __stdcall.

For reference, this is how SDL3 determines the calling convention:

https://github.com/libsdl-org/SDL/blob/72b7acfe8ac26498dac57d751e1b3485d30a62e3/include/SDL3/SDL_opengl.h#L92-L103

This is how KHRONOS_APIENTRY is determined:

irrlicht/source/Irrlicht/KHR/khrplatform.h

Lines 122 to 127 in 9b52d6f

	#if defined(_WIN32) && !defined(_WIN32_WCE) && !defined(KHRONOS_STATIC)
	/* Win32 but not WinCE */
	# define KHRONOS_APIENTRY __stdcall
	#else
	# define KHRONOS_APIENTRY
	#endif