image: add support for image/jpeg and image/png (#303)

* Copy from go1.17 image package
* Remove unnecessary files
* Reduce memory usage
* Add examples/ili9341/slideshow
* image: add ./image/README.md
* image: change convert2bin to . /cmd
* Makefile: add ./cmd to NOTEST

image/jpeg/callback.go

@@ -0,0 +1,19 @@
+package jpeg
+
+var (
+	callback    Callback = func(data []uint16, x, y, w, h, width, height int16) {}
+	callbackBuf []uint16
+)
+
+// A portion of the image data consisting of data, x, y, w, and h is passed to
+// Callback. The size of the whole image is passed as width and height.
+// If the callback is not called, add the implementation to
+// image/png.readImagePass.
+type Callback func(data []uint16, x, y, w, h, width, height int16)
+
+// SetCallback registers the buffer and fn required for Callback. Callback can
+// be called multiple times by calling Decode().
+func SetCallback(buf []uint16, fn Callback) {
+	callbackBuf = buf
+	callback = fn
+}