i965/fs: Import image format conversion primitives.

i965/fs: Import image format conversion primitives.

Define bitfield packing, unpacking and type conversion operations in
terms of which the image format conversion code will be implemented.
These don't directly know about image formats: The packing and
unpacking functions take a 4-tuple of bit shifts and a 4-tuple of bit
widths as arguments, determining the bitfield position of each
component.  Most of the remaining functions perform integer, fixed
point normalized, and floating point type conversions, mapping between
a target type with per-component bit widths given by a parameter and a
matching native representation of the same type.

v2: Drop VEC4 suport.
v3: Rebase.
v4: Fix clamping of negative floats in the unsigned case of
    emit_convert_to_scaled().

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>