Form CurrentFrame structure, rename frame_offset

Move a number of elements of AV1_COMMON which refer specifically to the
current frame into a new CurrentFrame structure.  Also, move those
elements specific to skip mode into a SkipModeInfo structure.  This
change is not exhaustive; we plan to move over more elements in future.

Also, frame_offset is renamed to order_hint in order to be clearer about
its purpose and to match the terminology in the bitstream spec.

This forms part of wider restructuring and refactoring in order to
achieve a clean API separation at the entry to the low-level encoder.

BUG=aomedia:2244

Change-Id: Ie85280138d11b83625a764602bef502252a4567f
diff --git a/av1/common/debugmodes.c b/av1/common/debugmodes.c
index 868f341..5242f19 100644
--- a/av1/common/debugmodes.c
+++ b/av1/common/debugmodes.c
@@ -17,7 +17,7 @@
 
 static void log_frame_info(AV1_COMMON *cm, const char *str, FILE *f) {
   fprintf(f, "%s", str);
-  fprintf(f, "(Frame %d, Show:%d, Q:%d): \n", cm->current_video_frame,
+  fprintf(f, "(Frame %d, Show:%d, Q:%d): \n", cm->current_frame.frame_number,
           cm->show_frame, cm->base_qindex);
 }
 /* This function dereferences a pointer to the mbmi structure