Fix T46143: Faces missing with GPU render

Epsilon was quite arbitrary for GPU, replaced with checking for zero-sized faces.

It should solve both original report and the new one. After the release we can check
why GPU doesn't produce accurate math here and go to the root of the issue.

intern/cycles/kernel/geom/geom_triangle_intersect.h

@@ -179,7 +179,7 @@ ccl_device_inline bool triangle_intersect(KernelGlobals *kg,
 		float4 a = tri_b - tri_a, b = tri_c - tri_a;
 		if(len_squared(make_float3(a.y*b.z - a.z*b.y,
 		                           a.z*b.x - a.x*b.z,
-		                           a.x*b.y - a.y*b.x)) < 1e-16f)
+		                           a.x*b.y - a.y*b.x)) == 0.0f)
 		{
 			return false;
 		}