Fix #35684: cycles unable to use full 6GB of memory on NVidia Titan GPU. We now

use arrays instead of textures for general storage on this card (image textures
are still stored as texture). Textures were found to be faster on older cards,
but the limits on 1D texture size have not increased along with the memory size,
which meant that the full 6 GB could not be used.

The performance actually seems to be slightly better with arrays in some tests
on Titan. For older cards there seems to be a bit of a mix, some are better and
others not. We may change those to use arrays too, but more testing is needed,
only Titan and Tesla K20 (sm_35) is changed for now.

The fact that arrays are faster is a bit surprising, as others found textures
to be faster on Kepler. However even if they were, the memory limitation is
more important to solve anyway.
https://research.nvidia.com/publication/understanding-efficiency-ray-traversal-gpus-kepler-and-fermi-addendum

intern/cycles/kernel/kernel_primitive.h

@@ -93,7 +93,11 @@ __device float3 primitive_tangent(KernelGlobals *kg, ShaderData *sd)
 {
 #ifdef __HAIR__
 	if(sd->segment != ~0)
+#ifdef __DPDU__
 		return normalize(sd->dPdu);
+#else
+		return make_float3(0.0f, 0.0f, 0.0f);
+#endif
 #endif
 
 	/* try to create spherical tangent from generated coordinates */
@@ -108,7 +112,11 @@ __device float3 primitive_tangent(KernelGlobals *kg, ShaderData *sd)
 	}
 	else {
 		/* otherwise use surface derivatives */
+#ifdef __DPDU__
 		return normalize(sd->dPdu);
+#else
+		return make_float3(0.0f, 0.0f, 0.0f);
+#endif
 	}
 }