add public function canShow() to be able to determine if show() would…

… block

In order to be able to minimize blocking this function is provided.

Adafruit_NeoPixel.cpp

@@ -79,7 +79,7 @@ void Adafruit_NeoPixel::show(void) {
   // subsequent round of data until the latch time has elapsed.  This
   // allows the mainline code to start generating the next frame of data
   // rather than stalling for the latch.
-  while((micros() - endTime) < 50L);
+  while(!canShow());
   // endTime is a private member (rather than global var) so that mutliple
   // instances on different pins can be quickly issued in succession (each
   // instance doesn't delay the next).

Adafruit_NeoPixel.h

@@ -64,6 +64,8 @@ class Adafruit_NeoPixel {
     Color(uint8_t r, uint8_t g, uint8_t b);
   uint32_t
     getPixelColor(uint16_t n) const;
+  inline bool
+    canShow(void) { return (micros() - endTime) >= 50L; }
 
  private: