add subsubsection styles & fix wrong PDF bookmarks

[fang-d]

1. Add correct heading spacing and type for the subsubsection in LaTeX.
2. Fix the wrong PDF bookmarks by removing the prefix dot.
3. Fix the hyperref warning "Token not allowed in a PDF string (Unicode)".

CC: @taiya, @cr333.

Thanks for considering my pull request.

[cr333]

Thank you @fang-d for these great fixes, much appreciated! :)

Could you please make the following two changes?

1. Adjust the font size of \subsubsections to 10pt, as per Section 2.2 of the Author Guidelines?
   You can add \font\tenbf = ptmb scaled 1000 before \font\elvbf, and use \tenbf for subsubsections.

2. Tweak the vertical spacing of headings a little. I suggest using the following:

   \def\cvprsubsection{\@startsection {subsection}{2}{\z@}{-8pt plus -2pt minus -2pt}{5pt} {\elvbf}}
   [...]
   \def\cvprsubsubsection{\@startsection {subsubsection}{3}{\z@}{-6pt plus -2pt minus -2pt}{3pt} {\tenbf}}

Example

[fang-d]

Hi @cr333! Thank you for your precise code review!

I have updated my pull request based on your valuable suggestions. Please reconsider my pull request and feel free to ask me to make further changes if this commit does not meet all the requirements.

[cr333]

Thank you, that's great! I'll go ahead and merge it

PS: It would be easier to follow your changes if you just continue committing to the branch this PR is created from, instead of force-pushing and overwriting the existing commit.

[fang-d]

Thank you very much for your suggestion and code review, I will keep this in mind in the future.