[FL-878] Gui: screen streaming (#337)

* GUI: canvas streaming
* Fix right status bar icon alignment

applications/gui/canvas_i.h

@@ -31,6 +31,18 @@ void canvas_reset(Canvas* canvas);
  */
 void canvas_commit(Canvas* canvas);
 
+/*
+ * Get canvas buffer.
+ * @return pointer to buffer
+ */
+uint8_t* canvas_get_buffer(Canvas* canvas);
+
+/*
+ * Get canvas buffer size.
+ * @return size of canvas in bytes
+ */
+size_t canvas_get_buffer_size(Canvas* canvas);
+
 /*
  * Set drawing region relative to real screen buffer
  */