fix: wrong hex color values (#2985)

Closes #2983

## Changes:

- fix `Color.toHex()` and `Color._componentToHex()` to handle fractional numbers, negative numbers and colors with transparency
- add tests for `Color.toHex()`

CHANGELOG.md

@@ -85,6 +85,7 @@ will be positioned with the top left of the graphic at the actor's position.
 - Fixed issue where Firefox on Linux would throw an error when using custom Materials due to unused attributes caused by glsl compiler optimization. 
 - Fixed issue where start transition did not work properly if deferred
 - Fixed issue where transitions did not cover the whole screen if camera was zoomed
+- Fixed issue where `Color.toHex()` produced invalid strings if the channel values are negative or fractional, or if the alpha channel was different than 1
 
 ### Updates
 

src/engine/Color.ts

@@ -222,15 +222,20 @@ export class Color {
    * @see https://stackoverflow.com/questions/5623838/rgb-to-hex-and-hex-to-rgb
    */
   private _componentToHex(c: number) {
-    const hex = c.toString(16);
+    // Handle negative and fractional numbers
+    const hex = Math.max(Math.round(c), 0).toString(16);
     return hex.length === 1 ? '0' + hex : hex;
   }
 
   /**
    * Return Hex representation of a color.
    */
   public toHex() {
-    return '#' + this._componentToHex(this.r) + this._componentToHex(this.g) + this._componentToHex(this.b);
+    let hexRepresentation = '#' + this._componentToHex(this.r) + this._componentToHex(this.g) + this._componentToHex(this.b);
+    if (this.a !== 1) {
+      hexRepresentation += this._componentToHex(this.a * 255);
+    }
+    return hexRepresentation;
   }
 
   /**
@@ -413,7 +418,7 @@ export class Color {
  * http://axonflux.com/handy-rgb-to-hsl-and-rgb-to-hsv-color-model-c
  */
 class HSLColor {
-  constructor(public h: number, public s: number, public l: number, public a: number) {}
+  constructor(public h: number, public s: number, public l: number, public a: number) { }
 
   public static hue2rgb(p: number, q: number, t: number): number {
     if (t < 0) {

src/spec/ColorSpec.ts

@@ -108,6 +108,17 @@ describe('A color', () => {
     expect(color.a).toBe(31 / 255);
   });
 
+  it('generate valid hex representation', () => {
+    color = ex.Color.White;
+    expect(color.toHex()).toBe('#ffffff');
+    color = ex.Color.Green;
+    expect(color.toHex()).toBe('#00ff00');
+    color = ex.Color.fromRGB(17,17,17,0.1);
+    expect(color.toHex()).toBe('#1111111a');
+    color = ex.Color.fromRGB(16.9, 16.9, 16.9);
+    expect(color.toHex()).toBe('#111111');
+  });
+
   it('can be darkened', () => {
     color = ex.Color.White.clone();
     color = color.darken();