util: Code generate functions to pack and unpack a single pixel.

util: Code generate functions to pack and unpack a single pixel.

Should work correctly for all pixel formats except SRGB formats.

Generated code made much simpler by defining the pixel format as
a C structure. For example this is the generated structure for
PIPE_FORMAT_B6UG5SR5S_NORM:

union util_format_b6ug5sr5s_norm {
   uint16_t value;
   struct {
      int r:5;
      int g:5;
      unsigned b:6;
   } chan;
};

Not used everywhere yet because it seems compiled code is slower than
bitshift arithmetic by some misterious reason. So we should generate
bitshift arithmetic at least for the simple UNORM pixel formats.