Support theme width and height offsets

Current use condition is LG V20, where the secondary screen is
actually just a corner chunk of the main screen.
In this case, we need to shift the UI down some pixels in order
to see it all, but just using Y offsets resulted in losing
the rest of the GUI below.

Example in BoardConfig.mk:
  # Shift TWRP off the secondary screen
  TW_Y_OFFSET := 160
  TW_H_OFFSET := -160

Change-Id: I5a06638ba0d05e5b9fd8a6245c630d6ea3394f78
diff --git a/gui/objects.hpp b/gui/objects.hpp
index 2a95022..7506277 100644
--- a/gui/objects.hpp
+++ b/gui/objects.hpp
@@ -42,6 +42,12 @@
 #ifndef TW_Y_OFFSET
 #define TW_Y_OFFSET 0
 #endif
+#ifndef TW_W_OFFSET
+#define TW_W_OFFSET 0
+#endif
+#ifndef TW_H_OFFSET
+#define TW_H_OFFSET 0
+#endif
 
 class RenderObject
 {