Fixed bug in PlasmaAnimation.ino

LEDS_PER_STRIP was artificially high using up extra space.  With 60 columns and 16 rows that means that you have 2 rows per strip and each strip would be 120 LEDs long. 60*16/8 = 120px per strip.  8x 120px strips = 960px total.

The size of the display and drawing buffers is accurately sized by a multiple of 6 (so I can see why this bug was introduced)