mesa/format_utils: Add a general format conversion function

mesa/format_utils: Add a general format conversion function

Most format conversion operations required by GL can be performed by
converting one channel at a time, shuffling the channels around, and
optionally filling missing channels with zeros and ones.  This adds a
function to do just that in a general, yet efficient, way.

v2:
 * Add better comments including full docs for functions
 * Don't use __typeof__
 * Use inline helpers instead of writing out conversions by hand,
 * Force full loop unrolling for better performance

v3: Add another set of parens around the MAX_INT macro

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>