CHAR(n) in ROW_FORMAT=REDUNDANT tables is always fixed-length
(n*mbmaxlen bytes), but in the temporary file it is variable-length
(n*mbminlen to n*mbmaxlen bytes) for variable-length character sets,
such as UTF-8.

The temporary file format used during index creation and online ALTER
TABLE is based on ROW_FORMAT=COMPACT. Thus, it should use the
variable-length encoding even if the base table is in
ROW_FORMAT=REDUNDNAT.

dtype_get_fixed_size_low(): Replace an assertion-like check with a
debug assertion.

rec_init_offsets_comp_ordinary(), rec_convert_dtuple_to_rec_comp():
Make this an inline function.  Replace 'ulint extra' with 'bool temp'.

rec_get_converted_size_comp_prefix_low(): Renamed from
rec_get_converted_size_comp_prefix(), and made inline. Add the
parameter 'bool temp'. If temp=true, do not add REC_N_NEW_EXTRA_BYTES.

rec_get_converted_size_comp_prefix(): Remove the comment about
dict_table_is_comp(). This function is only to be called for other
than ROW_FORMAT=REDUNDANT records.

rec_get_converted_size_temp(): New function for computing temporary
file record size. Omit REC_N_NEW_EXTRA_BYTES from the sizes.

rec_init_offsets_temp(), rec_convert_dtuple_to_temp(): New functions,
for operating on temporary file records.

rb:1559 approved by Jimmy Yang