Graphics: Optimize Viewport effect rendering

Using the kitchen sink plane shader for viewport effects, even
if only a small part of them are active, incurs great performance
loss on mobile, so split the rendering into multiple optional
passes which additionally use the blending hardware for faster
mixing (lerping).
Also, don't mirror the PingPong textures if the viewport effect
covers the entire screen area anyway.

shader/flatColor.frag

@@ -0,0 +1,7 @@
+
+uniform lowp vec4 color;
+
+void main()
+{
+	gl_FragColor = color;
+}

shader/gray.frag

@@ -0,0 +1,19 @@
+
+uniform sampler2D texture;
+uniform lowp float gray;
+
+varying vec2 v_texCoord;
+
+const vec3 lumaF = vec3(.299, .587, .114);
+
+void main()
+{
+	/* Sample source color */
+	vec4 frag = texture2D(texture, v_texCoord);
+
+	/* Apply gray */
+	float luma = dot(frag.rgb, lumaF);
+	frag.rgb = mix(frag.rgb, vec3(luma), gray);
+
+	gl_FragColor = frag;
+}

shader/minimal.vert

@@ -0,0 +1,8 @@
+
+uniform mat4 projMat;
+attribute vec2 position;
+
+void main()
+{
+	gl_Position = projMat * vec4(position, 0, 1);
+}