mesa: Add an implementation of a master convert function.

v2 by Iago Toral <[email protected]>:

- When testing if we can directly pack we should use the src format to check
  if we are packing from an RGBA format. The original code used the dst format
  for the ubyte case by mistake.
- Fixed incorrect number of bits for dst, it was computed using the src format
  instead of the dst format.
- If the dst format is an array format, check if it is signed. We were only
  checking this for the case where it was not an array format, but we need
  to know this in both scenarios.
- Fixed incorrect swizzle transform for the cases where we convert between
  array formats.
- Compute is_signed and bits only once and for the dst format. We were
  computing these for the src format too but they were overwritten by the
  dst values immediately after.
- Be more careful when selecting the integer path. Specifically, check that
  both src and dst are integer types. Checking only one of them should suffice
  since OpenGL does not allow conversions between normalized and integer types,
  but putting extra care here makes sense and also makes the actual requirements
  for this path more clear.
- The format argument for pack functions is the destination format we are
  packing to, not the source format (which has to be RGBA).
- Expose RGBA8888_* to other files. These will come in handy when in need to
  test if a given array format is RGBA or in need to pass RGBA formats to
  mesa_format_convert.

v3 by Samuel Iglesias <[email protected]>:

- Add an RGBA8888_INT definition.

v4 by Iago Toral <[email protected]> after review by Jason Ekstrand:

- Added documentation for _mesa_format_convert.
- Added additional explanatory comments for integer conversions.
- Ensure that we use _messa_swizzle_and_convert for all signed source formats.
- Squashed: do not directly (un)pack to RGBA UINT if the source is not unsigned.

v5 by Iago Toral <[email protected]>:

- Adapted to the new implementation of mesa_array_format as a plain uint32_t
  bitfield.

Reviewed-by: Jason Ekstrand <[email protected]>

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