Fixed signed/unsigned types comparison

BUG=18368388

Change-Id: I3f2927fd68e75a8fb5abde1b25e81416862076fc
Reviewed-on: https://swiftshader-review.googlesource.com/1474
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/Shader/PixelShader.cpp b/src/Shader/PixelShader.cpp
index ddddb94..9e4cff5 100644
--- a/src/Shader/PixelShader.cpp
+++ b/src/Shader/PixelShader.cpp
@@ -26,7 +26,7 @@
 
 		if(ps)   // Make a copy
 		{
-			for(int i = 0; i < ps->getLength(); i++)
+			for(size_t i = 0; i < ps->getLength(); i++)
 			{
 				append(new sw::Shader::Instruction(*ps->getInstruction(i)));
 			}