[x264] make threads configurable

[metalefty]

At the moment, the number of threads is hardcoded to 1. I should have made this configurable via gfx.toml. In a use case of an enterprise user, the server host lacks GPU but much more CPU cores are available than an ordinary desktop. This is very informative to me.

In most cases, 1-core is enough for the encoding but it's a good thing to make it configurable for flexibility.

xrdp/xrdp/xrdp_encoder_x264.c

Lines 153 to 160 in 2a17023

	x264_param_default_preset(&(xe->x264_params),
	xg->x264_param[ct].preset,
	xg->x264_param[ct].tune);
	xe->x264_params.i_threads = 1;
	xe->x264_params.i_width = (width + 15) & ~15;
	xe->x264_params.i_height = (height + 15) & ~15;
	xe->x264_params.i_fps_num = xg->x264_param[ct].fps_num;
	xe->x264_params.i_fps_den = xg->x264_param[ct].fps_den;

[metalefty]

I'm not sure if the number of OpenH264's threads is configurable but I would like to make it configurable as well if possible.