Use 4 colors in Checkered test shader

util/shader_utils.cpp

@@ -405,13 +405,29 @@ varying vec4 v_position;
 
 void main()
 {
-    if (v_position.x * v_position.y > 0.0)
+    bool isLeft = v_position.x < 0.0;
+    bool isTop = v_position.y < 0.0;
+    if (isLeft)
     {
-        gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
+        if (isTop)
+        {
+            gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
+        }
+        else
+        {
+            gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
+        }
     }
     else
     {
-        gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);
+        if (isTop)
+        {
+            gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);
+        }
+        else
+        {
+            gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
+        }
     }
 })";
 }

util/shader_utils.h

@@ -79,8 +79,13 @@ ANGLE_UTIL_EXPORT const char *Texture2D();
 namespace fs
 {
 
-// A shader that renders a simple checker pattern of red and green. X axis and y axis separate the
-// different colors. Needs varying v_position.
+// A shader that renders a simple checker pattern of red, green and blue. X axis and Y axis separate
+// the different colors. Needs varying v_position.
+//
+// - X < 0 && y < 0: Red
+// - X < 0 && y >= 0: Green
+// - X >= 0 && y < 0: Blue
+// - X >= 0 && y >= 0: Yellow
 ANGLE_UTIL_EXPORT const char *Checkered();
 
 // A shader that fills with color taken from uniform named "color".