core: port gradient shader to GL ES platforms.

src/gradient.lolfx

@@ -40,8 +40,31 @@ mat4 cluster = mat4(12.0,  5.0,  6.0, 13.0,
 void main()
 {
     vec4 col = pass_Color;
+#if defined GL_ES
+    int dx = int(mod(gl_FragCoord.x, 4.0));
+    int dy = int(mod(gl_FragCoord.y, 4.0));
+    /* Cluster */
+    float t;
+    if (dx == 0)
+    {
+        if (dy == 0) t = cluster[0][0]; else if (dy == 1) t = cluster[0][1]; else if (dy == 2) t = cluster[0][2]; else t = cluster[0][3];
+    }
+    else if (dx == 1)
+    {
+        if (dy == 0) t = cluster[1][0]; else if (dy == 1) t = cluster[1][1]; else if (dy == 2) t = cluster[1][2]; else t = cluster[1][3];
+    }
+    else if (dx == 2)
+    {
+        if (dy == 0) t = cluster[2][0]; else if (dy == 1) t = cluster[2][1]; else if (dy == 2) t = cluster[2][2]; else t = cluster[2][3];
+    }
+    else
+    {
+        if (dy == 0) t = cluster[3][0]; else if (dy == 1) t = cluster[3][1]; else if (dy == 2) t = cluster[3][2]; else t = cluster[3][3];
+    }
+#else
     float t = cluster[int(mod(gl_FragCoord.x, 4.0))]
                      [int(mod(gl_FragCoord.y, 4.0))];
+#endif
     t = (t + 0.5) / 17.0;
     col.x += fract(t - col.x) - t;
     col.y += fract(t - col.y) - t;