struct FragmentData {
    vec2 texCoords;
    vec4 color;
};

struct VertexData {
    vec2 pos;
    vec2 dir;
    vec4 color;
    int  texture;
};

const float SHIP_SIZE  = 25.0; // absolute ship size
const float GLOW_SIZE  =  4.0; // relative to ship size
const float GLOW_ALPHA =  0.5;