Rcheevos

[NikosSiak]

This PR adds support for rcheevos to RetroPlayer. This PR is used by xbmc/xbmc#20913 and depends on it to be build

[garbear]

hash should only by set if res == RC_OK. However, on the note of checking return value, here's a snippet of (maybe outdated) documentation I found:

---

Return value

If the function succeeds, the return value is RC_OK. Otherwise, the error code can be converted to a string using rc_error_str.

• RC_INVALID_STATE - one or more required parameters was not provided.
• RC_OUT_OF_MEMORY - enough memory could not be allocated to complete the operation.

---

To be fully correct, we should handle all pre-conditions and post-conditions when calling into an external library. To satisfy post-conditions, this means checking the result and logging errors.

[NikosSiak]

The comments above the function state:

/* generates a hash from a file.
   * returns non-zero on success, or zero on failure.
   */

That's why I didn't use the RC_OK you suggested in the previous comment, it was a bit confusing to check if the result is different from the OK value my thoughts when reading that were that if CCheevos::GenerateHashFromFile returns true it means it failed

[garbear]

I scrutinized the code and found a couple suggestions.

[garbear]

I tried compiling on Windows and found the following warnings:

[ 11%] Building CXX object CMakeFiles/game.libretro.dir/src/cheevos/Cheevos.cpp.obj
Cheevos.cpp(242): warning C4267: '-=': conversion from 'size_t' to 'unsigned int', possible loss of data
Cheevos.cpp(245): warning C4267: '=': conversion from 'size_t' to 'unsigned int', possible loss of data
Cheevos.cpp(248): warning C4267: '-=': conversion from 'size_t' to 'unsigned int', possible loss of data
Cheevos.cpp(250): warning C4267: '+=': conversion from 'size_t' to 'unsigned int', possible loss of data

[ 25%] Building CXX object CMakeFiles/game.libretro.dir/src/libretro/MemoryMap.cpp.obj
MemoryMap.cpp(14): warning C4018: '<': signed/unsigned mismatch

[garbear]

Just pointing out, the last two commits should be squashed so that the build succeeds on every commit to aid in bisecting. But no need to update for this PR.