Update getimagedata.js

Using a 32-Bit typed array, the iteration and access to the array is way faster. 
To access the values, you have to shift the values by it's 32-8-bit-relative position, as following:

red = data[0] & 0xFF
green = data[0] >> 8 & 0xFF
blue = data[0] >> 16 & 0xFF
alpha = data[0] >> 24 & 0xFF