Codechange: Template input buffer type in audio mixer.

[PeterN]

Motivation / Problem

The mix_int16 and mix_int8_to_int16 audio mixer functions are quite long and are very similar, save for a type difference and a shift factor.

Description

Add the type and the shift factor as template parameters to mix_int16 (int16 is the output type here which does not change) to reduce code duplication.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker