Expose horizontal model flipping to QC

[Toodles2You]

Lets QC set the Quake II render flag that flips models for left-handed weapons. pr_dumpplatform defines RF_XFLIP. Spoike currently recommends using RF_USEAXIS to flip models but, that requires a custom shader to disable face culling. I figured this was easier and cleaner.

static void() addviewmodel =
{
	float hand = getplayerkeyfloat (player_localnum, "hand");

	if (hand == 2)
		return;

	viewmodel.modelindex = getstatf (STAT_WEAPONMODELI);

	if (!viewmodel.modelindex)
		return;

	viewmodel.frame = getstatf (STAT_WEAPONFRAME);

	if (hand == 1)
		viewmodel.renderflags |= RF_XFLIP;
	else
		viewmodel.renderflags &= ~RF_XFLIP;

	addentity (viewmodel);
}