Correct reciprocal approximation for power-of-two values.

Intel's reciprocal approximation instruction is not exact for power-of-two
values. It provides 12 bits of mantissa precision and keeps a balance between
positive and negative errors, but the reciprocal of 2^x is not 2^-x. This
affects conformance tests which expect varyings not to be affected by the
perspective division. Correct for this by multiplying by the inverse.

Bug 27165393

Change-Id: Ie52ec511a14a4f447adc47ce9c875bbad03cd274
Reviewed-on: https://swiftshader-review.googlesource.com/4903
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>

src/Shader/ShaderCore.cpp

diff --git a/src/Shader/ShaderCore.cpp b/src/Shader/ShaderCore.cpp
index ad9f426..f177a68 100644
--- a/src/Shader/ShaderCore.cpp
+++ b/src/Shader/ShaderCore.cpp
@@ -271,7 +271,7 @@
 		return exponential2(log, pp);
 	}
 
-	Float4 reciprocal(RValue<Float4> x, bool pp, bool finite)
+	Float4 reciprocal(RValue<Float4> x, bool pp, bool finite, bool exactAtPow2)
 	{
 		Float4 rcp;
 
@@ -281,7 +281,7 @@
 		}
 		else
 		{
-			rcp = Rcp_pp(x);
+			rcp = Rcp_pp(x, exactAtPow2);
 
 			if(!pp)
 			{