Tweak fullscreen texture shaders

shader/texture/grayscale.frag

@@ -0,0 +1,12 @@
+#version 430 core
+in vec2 vTexPos;
+out vec4 fColor;
+
+uniform sampler2D screenTexture;
+
+void main()
+{
+    fColor = texture(screenTexture, vTexPos);
+    float av = (fColor.r + fColor.g + fColor.b) / 3.0;
+    fColor = vec4(av,av,av,1.0);
+}

shader/texture/simple.vert

@@ -0,0 +1,10 @@
+#version 430 core
+layout (location = 0) in vec2 pos;
+layout (location = 1) in vec2 texPos;
+out vec2 vTexPos;
+
+void main()
+{
+  gl_Position = vec4(pos,0,1);
+  vTexPos = texPos;
+}