rsx: Add floor workaround for GPUs with rounding issues

- Mainly affects nvidia where x/w * w can sometimes return a value smaller than x.
  In such conditions, floor(x) will return x-1 if x is an integer which is horribly wrong and exaggerates minor precision drift to great proportions.

rpcs3/Emu/RSX/Program/FragmentProgramDecompiler.h

@@ -294,6 +294,7 @@ public:
 	{
 		bool has_native_half_support = false;
 		bool emulate_depth_compare = false;
+		bool has_low_precision_rounding = false;
 	}
 	device_props;