Implement displaying of extra cursor color

kitty/utils.glsl

@@ -2,6 +2,8 @@
 #define zero_or_one(x) step(1.f, x)
 // condition must be zero or one. When 1 thenval is returned otherwise elseval
 #define if_one_then(condition, thenval, elseval) mix(elseval, thenval, condition)
+// a < b ? thenval : elseval
+#define if_less_than(a, b, thenval, elseval) mix(thenval, elseval, step(b, a))
 
 vec4 vec4_premul(vec3 rgb, float a) {
     return vec4(rgb * a, a);