refactor image resource loading code in minui

Reduce the number of copies of libpng boilerplate.  Rename
res_create_* functions to be more clear.  Make explicit the use of the
framebuffer pixel format for images, and handle more combinations of
input and output (eg, loading a grayscale image for display rather
than use as a text alpha channel).

Change-Id: I3d41c800a8f4c22b2f0167967ce6ee4d6b2b8846
diff --git a/screen_ui.h b/screen_ui.h
index ada006d..14b9138 100644
--- a/screen_ui.h
+++ b/screen_ui.h
@@ -64,6 +64,7 @@
   private:
     Icon currentIcon;
     int installingFrame;
+    const char* locale;
     bool rtl_locale;
 
     pthread_mutex_t updateMutex;