dx11: work around some float literal precision issue with the compiler

For some reason the directx compiler doesn't convert float literal in a
deterministic way (!). A difference of one ulp can be seen between
different compilation of the same code.
This causes an issue with the rounding of the alpha value for PT polys.
Issue #1207

core/rend/dx11/dx11_shaders.cpp

@@ -295,7 +295,7 @@ PSO main(in Pixel inpix)
 	#endif
 
 	#if cp_AlphaTest == 1
-		color.a = round(color.a * 255.0f) / 255.0f;
+		color.a = round(color.a * 255.0f) * 0.0039215688593685626983642578125; // 1 / 255
 		if (alphaTestValue > color.a)
 			discard;
 		color.a = 1.0f;

core/rend/dx11/oit/dx11_oitshaders.cpp

@@ -521,7 +521,7 @@ PSO main(in VertexIn inpix)
 	color *= trilinearAlpha;
 
 	#if cp_AlphaTest == 1
-		color.a = round(color.a * 255.0f) / 255.0f;
+		color.a = round(color.a * 255.0f) * 0.0039215688593685626983642578125; // 1 / 255
 		if (alphaTestValue > color.a)
 			discard;
 		color.a = 1.0f;