Constant SCREEN_VERTEX 

pub const SCREEN_VERTEX: &str = "#version 140
in vec3 position;
in vec2 texture_coords;
out vec2 uv;
void main() {
    uv = texture_coords;
    gl_Position = vec4(position, 1);
}";

Vertex shader that doesnt change position or uvs. takes in Vertex and TextureCoords and outputs uv. Intended for use with the screen mesh to draw over the screen. e.g. with the fxaa shader.