Make pkg-config errors non-fatal

Later configure tests will abort if the package is not found some other
way.
Cache render lookups
Store temporary rect locally
Remove write-only per2_flag
Remove unused r/g/b members of g3ds_vertex
Move debug-only digi functions into !RELEASE
Remove unused digi_set_volume
Remove unused digi_*_is_sound_playing
Fix !SDLMIXER build

Fixes: bd1c6d8c5da10c41adfe8c54b720f6136b853cf7
Make digi_max_channels const
Use range_for for SoundSlots
Use std::abort in Error

Error is called for fatal errors.  Use std::abort to get a core dump.