Swap X/y in images (#528)

* Swap X/Y in image encoding

* Fix PNG decoder

* Now really fix png

* reducing deps

* moving png.cpp into ev3

* bumped pxt

* updated shims

* fixing c++ compilation

* updated shims

* Fix pixel order

* update pxt

* Fix C++ image decoding

* Add expanded PNG images

* Generate E1 format images (in libs/ev3 run 'pxt buildsprites images')

* Go back to white background

* Remove PNG support

libs/ev3/images/meta.json

@@ -0,0 +1,8 @@
+{
+    "blockIdentity": "brick.__imagePicker",
+    "creator": "image.ofBuffer",
+    "star": {
+        "namespace": "images",
+        "mimeType": "image/x-mkcd-f1"
+    }
+}