Sanitize NaN/Inf in env cubemap and tonemap to stop one-frame black flashes

shaders/tonemapping.glsl

@@ -46,6 +46,17 @@ vec4 tonemap(vec4 x)
 	// with pureWhite=1.0 it collapses to identity (L*(1+L)/(1+L) = L) and
 	// just clips HDR>1.0 at the framebuffer -> washed-out / burnt look.
 	// ACES gives a smooth highlight shoulder + slight toe contrast.
-	return FBOUT(vec4(ACESFilm(x.rgb), x.a));
+
+	// Last-line-of-defense sanitize. ACESFilm has the form
+	//   (x*(a*x+b)) / (x*(c*x+d)+e)
+	// which maps NaN -> NaN and +Inf -> NaN (Inf/Inf). Either turns the
+	// pixel black after framebuffer clamp. A negative HDR input feeds a
+	// negative numerator/denominator and can produce non-physical output
+	// that also looks like a black flash. Clamp to a sensible range
+	// before the curve so a single bad upstream pixel can't escape.
+	vec3 hdr = x.rgb;
+	hdr = mix(hdr, vec3(0.0), vec3(any(isnan(hdr)) || any(isinf(hdr))));
+	hdr = max(hdr, vec3(0.0));
+	return FBOUT(vec4(ACESFilm(hdr), x.a));
 	//return FBOUT(vec4(reinhard(x.rgb), x.a));
 }