pngtosvg: Merge thresholding part in pngToBmp to sheettopng.py code to accommodate variable lighting in inputs

Related: #3

We should do this thresholding in sheettopng during the cropping process.

Lines 69 to 76 in adca270

    
           # Threshold image to convert each pixel to either black or white 
        
           threshold = 200 
        
           data = [] 
        
           for pix in list(img.getdata()): 
        
               if pix[0] >= threshold and pix[1] >= threshold and pix[3] >= threshold: 
        
                   data.append((255, 255, 255, 0)) 
        
               else: 
        
                   data.append((0, 0, 0, 1))

	# Threshold image to convert each pixel to either black or white
	threshold = 200
	data = []
	for pix in list(img.getdata()):
	if pix[0] >= threshold and pix[1] >= threshold and pix[3] >= threshold:
	data.append((255, 255, 255, 0))
	else:
	data.append((0, 0, 0, 1))