Tweak Controller::showLeds() to infer reversal from negative strip length

Example usage:

```
Show forward:

FastLED[0].setLeds(&leds[0], STRIP_LENGTH);
FastLED.show();

Show in reverse:

FastLED[0].setLeds(&leds[STRIP_LENGTH-1], -STRIP_LENGTH);
FastLED.show();

```

1 files changed, 5 insertions, 1 deletions

diff --git a/src/controller.h b/src/controller.h
index fe32d70d..1616eb91 100644
--- a/src/controller.h
+++ b/src/controller.h
@@ -404,7 +404,11 @@ protected:
     ///@param nLeds the number of leds being written out
     ///@param scale the rgb scaling to apply to each led before writing it out
     virtual void show(const struct CRGB *data, int nLeds, CRGB scale) {
-        PixelController<RGB_ORDER, LANES, MASK> pixels(data, nLeds, scale, getDither());
+        PixelController<RGB_ORDER, LANES, MASK> pixels(data, nLeds < 0 ? -nLeds : nLeds, scale, getDither());
+        if(nLeds < 0) {
+            // nLeds < 0 implies that we want to show them in reverse
+            pixels.mAdvance = -3;
+        }
         showPixels(pixels);
     }