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.

src/shader.h

@@ -87,6 +87,17 @@ protected:
 	GLint u_texSizeInv, u_translation;
 };
 
+class FlatColorShader : public ShaderBase
+{
+public:
+	FlatColorShader();
+
+	void setColor(const Vec4 &value);
+
+private:
+	GLint u_color;
+};
+
 class SimpleShader : public ShaderBase
 {
 public:
@@ -191,6 +202,17 @@ private:
 	GLint u_tone, u_color, u_flash, u_opacity;
 };
 
+class GrayShader : public ShaderBase
+{
+public:
+	GrayShader();
+
+	void setGray(float value);
+
+private:
+	GLint u_gray;
+};
+
 class TilemapShader : public ShaderBase
 {
 public:
@@ -285,6 +307,7 @@ private:
 /* Global object containing all available shaders */
 struct ShaderSet
 {
+	FlatColorShader flatColor;
 	SimpleShader simple;
 	SimpleColorShader simpleColor;
 	SimpleAlphaShader simpleAlpha;
@@ -292,6 +315,7 @@ struct ShaderSet
 	AlphaSpriteShader alphaSprite;
 	SpriteShader sprite;
 	PlaneShader plane;
+	GrayShader gray;
 	TilemapShader tilemap;
 	FlashMapShader flashMap;
 	TransShader trans;