mesa: Autogenerate most of format_pack.c

mesa: Autogenerate most of format_pack.c

We were auto-generating it before.  The problem was that the autogeneration
tool we were using was called "copy, paste, and edit".  Let's use a more
sensible solution.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
v2 by Samuel Iglesias <siglesias@igalia.com>
- Remove format_pack.c as it is now autogenerated
- Add usage of INDENT_FLAGS in Makefile.am
- Remove trailing blank line

v3 by Samuel Iglesias <siglesias@igalia.com>
- Merge format_convert.py into format_parser.py
   - Adapt pack_*_* function generations
- Fix out-of-tree build

v4 by Samuel Iglesias <siglesias@igalia.com>
- _get_datatype() is now a helper function

v5 by Samuel Iglesias <siglesias@igalia.com>
- format_pack.c.mako is now format_pack.py, with the template code
  inlined. It now auto-generates format_pack.c
- Simplify Makefile.am change.
- Modify SConscript to build format_pack.c with scons.
- Remove run_mako.py
- Add format_pack.c to gitignore

v6 by Samuel Iglesias <siglesias@igalia.com>:
- Don't allow float to non-normalized integer format conversions.
- Add non-normalized formats support for ubyte packing functions. Merge
the previously separated patch.
- Add clamping for non-normalized integer formats in pack_ubyte*()

v7 by Samuel Iglesias <siglesias@igalia.com>:
- Add assert to check that sRGB formats are 8-bit size.

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