Add background shaders

shader/background.frag

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+#version 430 core
+in vec2 gTexPos;
+out vec4 fColor;
+
+uniform sampler2D aTexture;
+
+void main()
+{
+    fColor = texture(aTexture, gTexPos);
+}

shader/background.geom

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+#version 430 core
+layout (points) in;
+layout (triangle_strip, max_vertices = 4) out;
+in vec4 vParams [];
+in vec2 vWinSize [];
+out vec2 gTexPos;
+
+void main()
+{
+  vec2 cPos = vec2(vParams[0].x / 250 , vParams[0].y /250 );
+  float zoom = vParams[0].w;
+  float xoff = vWinSize[0].x / (500*zoom) ;
+  float yoff = vWinSize[0].y / (500*zoom) ;
+  gTexPos = vec2 (cPos.x + xoff,cPos.y + yoff);
+  gl_Position = vec4 (1,1,0.9,1);
+  EmitVertex();
+  gTexPos = vec2 (cPos.x + xoff,cPos.y - yoff);
+  gl_Position = vec4 (1,-1,0.9,1);
+  EmitVertex();
+  gTexPos = vec2 (cPos.x - xoff,cPos.y + yoff);
+  gl_Position = vec4 (-1,1,0.9,1);
+  EmitVertex();
+  gTexPos = vec2 (cPos.x - xoff,cPos.y - yoff);
+  gl_Position = vec4 (-1,-1,0.9,1);
+  EmitVertex();
+
+  EndPrimitive();
+}

shader/background.vert

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+#version 430 core
+layout (location = 0) in vec4 params;
+layout (location = 1) in vec2 winSize;
+out vec4 vParams;
+out vec2 vWinSize;
+void main()
+{
+  gl_Position = vec4(0,0,0,0);
+  vParams = params;
+  vWinSize = winSize;
+}