Fix cached JPEG test

This test didn't really check anything useful as the end result would be
the same if the second JPEG failed to render.

Fix this by clearing the canvas between the images, so we can tell if
the second image actually rendered or not.

tests/test.jpeg.js

@@ -246,6 +246,8 @@ describe('JPEG Decoder', function () {
 
         testDecodeRect(decoder, 0, 0, 4, 4, data1, display, 24);
 
+        display.fillRect(0, 0, 4, 4, [128, 128, 128, 255]);
+
         let data2 = [
             // JPEG data
             0xff, 0xd8, 0xff, 0xe0, 0x00, 0x10, 0x4a, 0x46,