Introduce generateRandomSeed function

src/Arduboy2.cpp

@@ -275,7 +275,7 @@ int Arduboy2Base::cpuLoad()
   return lastFrameDurationMs*100 / eachFrameMillis;
 }
 
-void Arduboy2Base::initRandomSeed()
+unsigned long Arduboy2Base::generateRandomSeed()
 {
   power_adc_enable(); // ADC on
 
@@ -283,9 +283,16 @@ void Arduboy2Base::initRandomSeed()
   ADCSRA |= _BV(ADSC); // start conversion (ADMUX has been pre-set in boot())
   while (bit_is_set(ADCSRA, ADSC)) { } // wait for conversion complete
 
-  randomSeed(((unsigned long)ADC << 16) + micros());
+  unsigned long seed = (static_cast<unsigned long>(ADC) << 16) + micros();
 
   power_adc_disable(); // ADC off
+  
+  return seed;
+}
+
+void Arduboy2Base::initRandomSeed()
+{
+  randomSeed(generateRandomSeed);
 }
 
 /* Graphics */

src/Arduboy2.h

@@ -692,6 +692,19 @@ class Arduboy2Base : public Arduboy2Core
    */
   uint8_t* getBuffer();
 
+  /** \brief
+   * Create a seed suitable for use with a random number generator.
+   *
+   * \details
+   * The seed is generated with a random value derived from entropy from an
+   * ADC reading of a floating pin combined with the microseconds since boot.
+   *
+   * This method is still most effective when called after a semi-random time,
+   * such as after a user hits a button to start a game or other semi-random
+   * event.
+   */
+  unsigned long generateRandomSeed();
+
   /** \brief
    * Seed the random number generator with a random value.
    *