mesa: Autogenerate format_unpack.c

mesa: Autogenerate format_unpack.c

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
v2 by Samuel Iglesias <siglesias@igalia.com>:
- Add usage of INDENT_FLAGS in Makefile.am

v3 by Samuel Iglesias <siglesias@igalia.com>:
- Modify unpack_float_*() and unpack_ubyte_*() function generation
to use c.datatype() and f.datatype()
- Fix out-of-tree build

v4 by Samuel Iglesias <siglesias@igalia.com>:
- format_unpack.c.mako is now format_unpack.py, with the template code
  inlined. It now auto-generates format_unpack.c
- Add format_unpack.c to gitignore.
- Simplify Makefile.am change
- Modify SConscript to build format_unpack.c with scons

v5 by Samuel Iglesias <siglesias@igalia.com>:
- Don't allow float to non-normalized integer format conversions.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>