samplePixels
Nearest-neighbour samples this bitmap down to at most sampleArea pixels.
Reads only the rows it keeps, so a 12 megapixel photo costs about 128 row reads and one 16k int array, never a full-size copy. Pass a sampleArea of zero or less to read every pixel.
Nearest neighbour is deliberate. Averaging would blend a vibrant pixel into the dull ones around it, and the vibrant pixels are the ones a theme wants.
Return
ARGB pixels in row-major order, plus the sampled width and height.
Parameters
sampleArea
The most pixels to keep, or zero and below to keep them all.