Anchor viewmodel FOV to vanilla under Hor+ widescreen

Under Hor+ widescreen (r_aspect > 0) the world is rendered with a
widened horizontal FOV.  The viewmodel is a screen-anchored prop, not
world geometry, so sharing that widening stretches the weapon toward
the screen edges -- increasingly obvious at 21:9 and 32:9.

Pin the viewmodel to the vanilla horizontal FOV (the authored scr_fov
at 4:3) by temporarily swapping the un-widened horizontal scale around
the alias draw in R_DrawViewModel.  Only xscale / aliasxscale /
xscaleinv depend on the widened horizontalFieldOfView; xcenter and
aliasxcenter derive from vrect.width alone and are aspect-invariant,
and the vertical scale is already anchored by the pixelAspect
compensation in R_ViewChanged.  Restoring just the horizontal scale
therefore reproduces the vanilla gun projection while leaving the
world untouched.

Verified: at 1920x1080 the world horizontal scale shrinks with wider
ratios (960 at 4:3 -> 720 at 16:9 -> 360 at 32:9) while the gun stays
pinned at the vanilla 960 at every ratio.  4:3 is a no-op.

common/r_main.c

@@ -1189,6 +1189,43 @@ R_DrawViewModel(void)
 
     r_viewlighting.plightvec = lightvec;
 
+    /* Viewmodel FOV anchor.
+     *
+     * Under Hor+ widescreen (r_aspect > 0) the world is rendered with a
+     * widened horizontal FOV.  The viewmodel is a screen-anchored prop,
+     * not world geometry, so applying that same widening stretches the
+     * weapon toward the screen edges at wide ratios (very visible at
+     * 21:9 / 32:9).  Other ports solve this by giving the viewmodel its
+     * own FOV; here we pin it to the vanilla horizontal FOV (the
+     * authored scr_fov at 4:3) by temporarily swapping in the
+     * un-widened horizontal scale around the alias draw.
+     *
+     * Only the horizontal scale depends on the widened
+     * horizontalFieldOfView; xcenter/aliasxcenter derive from
+     * vrect.width alone and are aspect-invariant, and the vertical
+     * scale is already anchored by the pixelAspect compensation in
+     * R_ViewChanged.  So restoring just xscale / aliasxscale / xscaleinv
+     * to their 4:3 values reproduces the vanilla gun projection while
+     * leaving the world untouched.  At 4:3 this is a no-op. */
+    if ((int)r_aspect.value > 0) {
+	float hfov43 = 2.0f * tanf(scr_fov.value / 360.0f * (float)M_PI);
+	float xscale43 = r_refdef.vrect.width / hfov43;
+	float save_xscale      = xscale;
+	float save_aliasxscale = aliasxscale;
+	float save_xscaleinv   = xscaleinv;
+
+	xscale      = xscale43;
+	aliasxscale = xscale43 * r_aliasuvscale;
+	xscaleinv   = 1.0f / xscale43;
+
+	R_AliasDrawModel(e, &r_viewlighting);
+
+	xscale      = save_xscale;
+	aliasxscale = save_aliasxscale;
+	xscaleinv   = save_xscaleinv;
+	return;
+    }
+
     R_AliasDrawModel(e, &r_viewlighting);
 }