gallium: Add a transcode_astc driconf option

This is similar to the transcode_etc flag in that it changes the ASTC
fallback (when present) to use DXT5 instead of RGBA8888.  This reduces
the memory footprint of the app at the expense of a bit of correctness.
Because it's not quite correct, it's hidden behind a driconf option.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10476>

src/gallium/include/frontend/api.h

@@ -242,6 +242,7 @@ struct st_config_options
    bool force_integer_tex_nearest;
    bool force_gl_names_reuse;
    bool transcode_etc;
+   bool transcode_astc;
    char *force_gl_vendor;
    unsigned char config_options_sha1[20];
 };