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/shims.d.ts

@@ -0,0 +1,6 @@
+// Auto-generated. Do not edit.
+
+
+declare interface Image {}
+
+// Auto-generated. Do not edit. Really.