swrast: Fix a warning from gcc 11

swrast: Fix a warning from gcc 11

gcc 11 dixit:

   In function ‘sample_2d_ewa’,
       inlined from ‘sample_lambda_2d_aniso’ at ../src/mesa/swrast/s_texfilter.c:1995:10:
   ../src/mesa/swrast/s_texfilter.c:1729:13: warning: ‘sample_2d_nearest’ reading 16 bytes from a region of size 8 [-Wstringop-overread]
    1729 |             sample_2d_nearest(ctx, samp, img, newCoord, rgba);
         |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   ../src/mesa/swrast/s_texfilter.c: In function ‘sample_lambda_2d_aniso’:
   ../src/mesa/swrast/s_texfilter.c:1729:13: note: referencing argument 4 of type ‘const GLfloat *’ {aka ‘const float *’}

Indeed, newCoord is GLfloat[2] but the argument is typed GLfloat[4],
even though only the first two (s and t) are ever read. Fix the array
size in the function signature to reflect the maximum element actually
addressed.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11273>