lcd/skin_engine: Add the ability to draw onto the backdrop layer

The framebuffer the lcd driver uses can now be changed on the fly
which means that regular lcd_* drawing functions can draw onto the
"backdrop" buffer. The skin engine can use this to create layered
effects.

Add the tag %VB to a viewport to draw that viewport onto the
backdrop layer. If you want to draw an image onto the backdrop
framebuffer use %x(backdrop filename) instead of %X() inside
a viewport with %VB.

Change-Id: I741498e2af6d4f2d78932cabe8942317893e7cfc

lib/skin_parser/tag_table.c

@@ -204,6 +204,7 @@ static const struct tag_info legal_tags[] =
     { SKIN_TOKEN_VIEWPORT_BGCOLOUR,       "Vb" , "s", SKIN_REFRESH_STATIC|NOBREAK },
     { SKIN_TOKEN_VIEWPORT_TEXTSTYLE,      "Vs" , "S|s", SKIN_REFRESH_STATIC },
     { SKIN_TOKEN_VIEWPORT_GRADIENT_SETUP, "Vg" , "SS|s", SKIN_REFRESH_STATIC|NOBREAK },
+    { SKIN_TOKEN_VIEWPORT_DRAWONBG,       "VB" , "", SKIN_REFRESH_STATIC|NOBREAK },
     
     { SKIN_TOKEN_VIEWPORT_CONDITIONAL,  "Vl" , "SIIiii", 0 },
     { SKIN_TOKEN_UIVIEWPORT_LOAD,       "Vi" , "sIIiii", 0 },

lib/skin_parser/tag_table.h

@@ -92,7 +92,8 @@ enum skin_token_type {
     SKIN_TOKEN_VIEWPORT_BGCOLOUR,
     SKIN_TOKEN_VIEWPORT_TEXTSTYLE,
     SKIN_TOKEN_VIEWPORT_GRADIENT_SETUP,
-    
+    SKIN_TOKEN_VIEWPORT_DRAWONBG,
+
     /* Battery */
     SKIN_TOKEN_BATTERY_PERCENT,
     SKIN_TOKEN_BATTERY_PERCENTBAR,