llvmpipe: add EXT_packed_float render target format support

New conversion code to handle conversion from/to r11g11b10 AoS to/from
SoA floats, and also add code for conversion from rgb9e5 AoS to float SoA
(which works pretty much the same as r11g11b10 except for the packing).
(This code should also be used for texture sampling instead of
relying on u_format conversion but it's not yet, so rgb9e5 is unused.)
Unfortunately a crazy amount of hacks is necessary to get the conversion
code running in llvmpipe's generate_unswizzled_blend, which isn't well
suited for formats where the storage representation has nothing to do
with what's needed for blending (moreover, the conversion will convert
from packed AoS values, which is the storage format, to float SoA values,
because this is much more natural for the conversion, and likewise from
SoA values to packed AoS values - but the "blend" (which includes
trivial things like partial mask) works on AoS values, so incoming fs
values will go SoA->AoS, values from destination will go packed
AoS->SoA->AoS, then do blend, then AoS->SoA->packed AoS which probably
isn't the most efficient way though the shuffles are probably bearable).

Passes piglit fbo-blending-formats (with GL_EXT_packed_float parameter),
still need to verify Inf/NaNs (where most of the complexity in the
conversion comes from actually).

v2: drop the (very bogus) rgb9e5 part, and do component extraction
in the helper code for r11g11b10 to float conversion, making the code
slightly more compact (suggested by Jose), now that there are no other
callers left this works quite well. (Could do the same for the
opposite way but it's less than ideal there, final part of packing
needs to be done in caller anyway and there'd be another conditional.)

v3: minor style and comment fixes. Also fix a potential issue with
negative zero being potentially returned by max(src, zero) as we
don't have well-defined min/max behavior (fortunately no additonal cost).

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

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