Add a shader window with sliders

[polomint]

Add a window that lets the user move sliders for the variables in the shader, and save/load them.

GLint           u_borderSize;
GLint           u_contrast;
GLint           u_saturation;
GLint           u_brightness;
GLint           u_scanlineSize;
GLint           u_scanlines;
GLint           u_screenCurve;
GLint           u_pixelFilterValue;
GLint           u_rgbOffset;
GLint           u_showVignette;
GLint           u_vignetteX;
GLint           u_vignetteY;
GLint           u_showReflection;
GLint           u_time;
GLint           u_screenSize;