Constant RECT_VERT_SRC 

pub const RECT_VERT_SRC: &str = r#"
#version 330 core

layout(location = 0) in vec2 v_pos;       // [0,1] unit quad

layout(location = 1) in vec2 i_position;   // top-left corner
layout(location = 2) in vec2 i_size;       // width, height
layout(location = 3) in vec4 i_color;

uniform mat4 u_projection;

out vec4 f_color;

void main() {
    vec2 world = i_position + v_pos * i_size;
    gl_Position = u_projection * vec4(world, 0.0, 1.0);
    f_color = i_color;
}
"#;

Vertex shader for solid-color rectangles.