StandardStyle : Discard fragments with zero texture alpha

We were already trying to discard fragments with zero alpha so that they don't affect selection hits. But we were doing that before taking the texture alpha into account. Although the original motivation was selection hits, discarding zero alpha also avoids depth buffer writes, which is important now that Backdrops use depth buffering to control the order of nesting.

src/GafferUI/StandardStyle.cpp

@@ -1499,11 +1499,6 @@ static const std::string &fragmentSource()
 		"		OUTCOLOR.a *= ieFilteredPulse( 0.2, 0.8, gl_TexCoord[0].x );"
 		"	}"
 
-		"	if( OUTCOLOR.a == 0.0 )"
-		"	{"
-		"		discard;"
-		"	}"
-
 		/// \todo Deal with all colourspace nonsense outside of the shader. Ideally the shader would accept only linear"
 		/// textures and output only linear data."
 
@@ -1524,6 +1519,11 @@ static const std::string &fragmentSource()
 		"		OUTCOLOR = vec4( OUTCOLOR.rgb, OUTCOLOR.a * texture2D( texture, gl_TexCoord[0].xy ).a );"
 		"	}\n"
 
+		"	if( OUTCOLOR.a == 0.0 )"
+		"	{"
+		"		discard;"
+		"	}\n"
+
 		"#if __VERSION__ >= 330\n"
 		"	ieCoreGLNameOut = ieCoreGLNameIn;\n"
 		"#endif\n"