llvmpipe: decrease MAX_SCENES from 2 to 1

Multiple scenes per context are meant to be used so a new scene can be built
while another one is processed in rasterization. However, quite surprisingly,
this does not actually work (and according to git log, possibly never did,
though maybe it did at some point further back (5 years+) but was buggy)
because we always wait immediately on the rasterizer to finish the scene when
contexts (and hence setup/scene) is flushed. This means when we try to get
an empty scene later, any old one is already empty again.
Thus using multiple scenes is just a waste of memory (not too bad, since the
additional scenes are guaranteed to be empty, which means their size ought to
be one data block (64kB) plus the size of some structs), without actually
really doing anything. (There is also quite some code for the whole concept of
multiple scenes which doesn't really do much in practice, but keep it hoping
the wait-on-scene-flush can be fixed some day.)

Reviewed-by: Jose Fonseca <[email protected]>

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