G/S/C Unicorn: Fix get_brightness to use correct max value.

Add a comment noting that 256 is the correct maximum brightness.

libraries/cosmic_unicorn/cosmic_unicorn.cpp

@@ -494,11 +494,14 @@ namespace pimoroni {
   void CosmicUnicorn::set_brightness(float value) {
     value = value < 0.0f ? 0.0f : value;
     value = value > 1.0f ? 1.0f : value;
+    // Max brightness is - in fact - 256 since it's applied with:
+    // result = (channel * brightness) >> 8
+    // eg: (255 * 256) >> 8 == 255
     this->brightness = floor(value * 256.0f);
   }
 
   float CosmicUnicorn::get_brightness() {
-    return this->brightness / 255.0f;
+    return this->brightness / 256.0f;
   }
 
   void CosmicUnicorn::adjust_brightness(float delta) {

libraries/galactic_unicorn/galactic_unicorn.cpp

@@ -488,11 +488,14 @@ namespace pimoroni {
   void GalacticUnicorn::set_brightness(float value) {
     value = value < 0.0f ? 0.0f : value;
     value = value > 1.0f ? 1.0f : value;
+    // Max brightness is - in fact - 256 since it's applied with:
+    // result = (channel * brightness) >> 8
+    // eg: (255 * 256) >> 8 == 255
     this->brightness = floor(value * 256.0f);
   }
 
   float GalacticUnicorn::get_brightness() {
-    return this->brightness / 255.0f;
+    return this->brightness / 256.0f;
   }
 
   void GalacticUnicorn::adjust_brightness(float delta) {

libraries/stellar_unicorn/stellar_unicorn.cpp

@@ -485,11 +485,14 @@ namespace pimoroni {
   void StellarUnicorn::set_brightness(float value) {
     value = value < 0.0f ? 0.0f : value;
     value = value > 1.0f ? 1.0f : value;
+    // Max brightness is - in fact - 256 since it's applied with:
+    // result = (channel * brightness) >> 8
+    // eg: (255 * 256) >> 8 == 255
     this->brightness = floor(value * 256.0f);
   }
 
   float StellarUnicorn::get_brightness() {
-    return this->brightness / 255.0f;
+    return this->brightness / 256.0f;
   }
 
   void StellarUnicorn::adjust_brightness(float delta) {