[client] egl: added new super+N binding to increase image gain

This feture is to allow the use of the key combination <super>+N to
increase the brightness of the screen when using monitors with poor
backlighting. Can help in some games.

N = Night vision

client/renderers/EGL/shader/desktop_rgb.frag

@@ -4,8 +4,12 @@ in  highp vec2 uv;
 out highp vec4 color;
 
 uniform sampler2D sampler1;
+uniform int nv;
+uniform highp float nvGain;
 
 void main()
 {
   color = texture(sampler1, uv);
+  if (nv == 1)
+    color *= nvGain;
 }