Doing so forces the compiler to actually emit storage for them in bss.
If we build the table as a local the compiler is allowed to optimize it
down to constants. Trades about 500 bytes of text for -6000 bytes of
bss:

   text	   data	    bss	    dec	    hex	filename
  16496	      0	   6146	  22642	   5872	build/src/mesa/bbe4a73@@mesa_common@sta/main_texstore.c.o
  17026	      0	      0	  17026	   4282	build/src/mesa/bbe4a73@@mesa_common@sta/main_texstore.c.o