mesa: fix indexing error in unpack_Z32_FLOAT_X24S8()

The source array elements are 8-bytes (float + uint) so we need
to multiply the src index by 2 to get the right array stride.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/format_unpack.c

@@ -661,7 +661,7 @@ unpack_Z32_FLOAT(const void *src, GLfloat dst[][4], GLuint n)
    for (i = 0; i < n; i++) {
       dst[i][0] =
       dst[i][1] =
-      dst[i][2] = s[i];
+      dst[i][2] = s[i * 2];
       dst[i][3] = 1.0F;
    }
 }