Add a workaround for skybox.

The problem was with shader-based pipeline with disabled "advanced effects" in options.

In this case we don't use RTTs and gl_FragCoord contains values in range of whole window. So fo 2 players we still get gl_FragCoord.y = 0..window_size instead of gl_FragCoord.y = 0..window_size/2.

The easiest way to solve it seems to be modulo it by current viewport size. It should be compatible with advanced pipeline as well as single-player games.

Atm. I'm not sure if this should be applied to 0.9.2 branch. It should work fine, but needs some testing.

data/shaders/sky.frag

@@ -4,7 +4,7 @@ out vec4 FragColor;
 
 void main(void)
 {
-    vec3 eyedir = vec3(gl_FragCoord.xy / screen, 1.);
+    vec3 eyedir = vec3(mod(gl_FragCoord.xy, screen) / screen, 1.);
     eyedir = 2.0 * eyedir - 1.0;
     vec4 tmp = (InverseProjectionMatrix * vec4(eyedir, 1.));
     tmp /= tmp.w;