Bit accounting.

This patch adds bit account infrastructure to the bit reader API.
When configured with --enable-accounting, every bit reader API
function records the number of bits necessary to decoding a symbol.
Accounting symbol entries are collected in global accounting data
structure, that can be used to understand exactly where bits are
spent (http://aomanalyzer.org). The data structure is cleared and
reused each frame to reduce memory usage. When configured without
--enable-accounting, bit accounting does not incur any runtime
overhead.

All aom_read_xxx functions now have an additional string parameter
that specifies the symbol name. By default, the ACCT_STR macro is
used (which expands to __func__). For more precise accounting,
these should be replaced with more descriptive names.

Change-Id: Ia2e1343cb842c9391b12b77272587dfbe307a56d

av1/decoder/decoder.c

@@ -126,6 +126,9 @@ AV1Decoder *av1_decoder_create(BufferPool *const pool) {
 #if CONFIG_LOOP_RESTORATION
   av1_loop_restoration_precal();
 #endif  // CONFIG_LOOP_RESTORATION
+#if CONFIG_ACCOUNTING
+  aom_accounting_init(&pbi->accounting);
+#endif
 
   cm->error.setjmp = 0;
 
@@ -154,6 +157,10 @@ void av1_decoder_remove(AV1Decoder *pbi) {
     av1_loop_filter_dealloc(&pbi->lf_row_sync);
   }
 
+#if CONFIG_ACCOUNTING
+  aom_accounting_clear(&pbi->accounting);
+#endif
+
   aom_free(pbi);
 }