Bug 1501523 - part 1 - externalize jpeg_nbits_table from jpeg_nbits_t…

…able.h; r=aosmond

The current setup means that including jpeg_nbits_table.h creates a 64K
static table in the current compilation unit...for every inclusion of
jpeg_nbits_table.h.  Since we have two such inclusions, we have two
copies of this table, and the linker is not merging them; perhaps there
is a limit on the size of identical blobs of data it will merge.

In any event, this setup is suboptimal; instead, we should have a single
instance of jpeg_nbits_table in some external source file, and the
header can declare it like any other variable.