mesa: Do something sensible when on-line compression is requested but not possible

It is possible to force S3TC extensions to be enabled.  This is
generally done to support applications that will only supply
pre-compressed textures.  This accounts for the vast majority of
applications.

However, there is still the possibility of an application asking for
on-line compression.  In that case, generate a warning and substitute a
generic compressed format.  The driver will either pick an uncompressed
format or a compressed format that Mesa can handle on-line (e.g., FXT1).

This should only cause problems for applications that request on-line
compression and read the compressed texture back.  This is likely an
infinitesimal subset of an already infinitesimal subset.

Signed-off-by: Ian Romanick <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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