arch(draw): allow replacing the draw engine

BREAKING CHANGE: the API of lv_draw_... function have been changed

examples/libs/ffmpeg/lv_example_ffmpeg_2.c

@@ -1,5 +1,6 @@
 #include "../../lv_examples.h"
-#if LV_USE_FFMPEG && LV_BUILD_EXAMPLES
+#if LV_BUILD_EXAMPLES
+#if LV_USE_FFMPEG
 
 /**
  * Open a video from a file
@@ -28,3 +29,4 @@ void lv_example_ffmpeg_2(void)
 }
 
 #endif
+#endif