Add a framebuffer to hold a texture

shaders/invert_screen.frag

@@ -0,0 +1,15 @@
+#version 330 core
+out vec4 FragColor;
+
+in vec2 TexCoords;
+
+uniform float u_time;
+uniform uint u_frame;
+uniform vec2 u_resolution;
+uniform vec2 u_mouse;
+uniform sampler2D u_texture;
+
+void main() {
+  //FragColor = texture(u_texture, TexCoords);
+  FragColor = vec4(1.0 - texelFetch(u_texture, ivec2(gl_FragCoord), 0));
+}