Add a SpriteBatch shader for brightness & contrast

[crykn]

Create a new default shader for the sprite batch that allows specifying the brightness & contrast.

• Starting point https://github.com/mattdesl/lwjgl-basics/wiki/LibGDX-Brightness-&-Contrast & here
• Find advanced shader implementations: maybe see here or here:

  #ifdef GL_ES
  #define LOWP lowp
  precision mediump float;
  #else
  #define LOWP 
  #endif
  varying vec2 v_texCoords;
  varying LOWP vec4 v_color;
  uniform sampler2D u_texture;
  vec3 applyHue(vec3 aColor, float aHue)
  {
  float angle = aHue;
  vec3 k = vec3(0.57735);
  float c = cos(angle);
  //Rodrigues' rotation formula
  return aColor * c + cross(k, aColor) * sin(angle) + k * dot(k, aColor) * (1.0 - c);
  }
  void main()
  {
  float hue = 6.2831853 * (v_color.r - 0.5);
  float saturation = v_color.g * 2.0;
  float brightness = v_color.b - 0.5;
  float contrast = pow(v_color.a + 0.5, 1.709);
  float lightFix = 1.0 + pow(contrast, 1.41421356);
  vec4 tgt = texture2D( u_texture, v_texCoords );
  tgt.rgb = applyHue(tgt.rgb, hue);
  vec3 ycc = vec3(
   (0.5 * pow(dot(tgt.rgb, vec3(0.375, 0.5, 0.125)), contrast) * lightFix + brightness), // lightness
   ((tgt.r - tgt.b) * saturation), // warmth
   ((tgt.g - tgt.b) * saturation)); // mildness
  gl_FragColor = clamp(vec4(
   dot(ycc, vec3(1.0, 0.625, -0.5)), // back to red
   dot(ycc, vec3(1.0, -0.375, 0.5)), // back to green
   dot(ycc, vec3(1.0, -0.375, -0.5)), // back to blue
   tgt.a), 0.0, 1.0); // keep alpha, then clamp
  }

[crykn]

After further thought, a brightness & contrast shader is a lot less useful than anticipated considering the fact that normal rendering may need a different shader as well and then the brightness setting of the default shader will be ignored.