Support overwriting an arbitrary number of literal bits.

Overwriting was only guaranteed to work if a whole number
of bytes were being overwritten.

Change-Id: I5e72cb337ec6ff691e93288de9f751b583654a17

aom_dsp/bitwriter_buffer.c

@@ -23,12 +23,8 @@ void aom_wb_write_bit(struct aom_write_bit_buffer *wb, int bit) {
   const int off = (int)wb->bit_offset;
   const int p = off / CHAR_BIT;
   const int q = CHAR_BIT - 1 - off % CHAR_BIT;
-  if (q == CHAR_BIT - 1) {
-    wb->bit_buffer[p] = bit << q;
-  } else {
-    wb->bit_buffer[p] &= ~(1 << q);
-    wb->bit_buffer[p] |= bit << q;
-  }
+  wb->bit_buffer[p] &= ~(1 << q);
+  wb->bit_buffer[p] |= bit << q;
   wb->bit_offset = off + 1;
 }