[GML] Calculate if enemy is on the right or right side of our instance

side.gml

// calcualte position difference (a.k.a. tangent vector):
var dx = enemy.x - x;
var dy = enemy.y - y;

// calculate angle in radians of instance:
var rad = degtorad(image_angle);

// calculate where is instance right side (a.k.a. binormal vector):
var bx = -sin(rad);
var by = cos(rad);

// calculate on which side of instance enemy is (1 - on the right; 0 - directly in front; -1; on the left):
var side = sign(dot_product(dx, dy, bx, by));