Convert pixels to valid on export

lib.py

@@ -6,6 +6,12 @@ class Pixel(namedtuple('Pixel', ['r', 'g', 'b'])):
 			raise ValueError('Some component too bright.')
 		if min(self.r, self.g, self.b) < 0:
 			raise ValueError('Some component negative.')
+	def _to_valid(self):
+		return Pixel(
+			int(max(0, min(255, self.r))),
+			int(max(0, min(255, self.g))),
+			int(max(0, min(255, self.b))),
+		)
 	def __add__(self, other):
 		return Pixel(
 			self.r + other.r,
@@ -58,7 +64,7 @@ class Picture:
 		seq = []
 		for line in self.pixels:
 			for pix in line:
-				seq.extend(pix)
+				seq.extend(pix._to_valid())
 		
 		# bytes…
 		b = bytes(seq)