Preserve signedness from opt_encoder_handler for scroll data on Ploopy devices

[sbuller]

This is a bugfix for Ploopy devices (trackball and mouse).

Ploopy devices use an optical encoder to produce scroll information. The opt_encoder_handler returns a value of 1, -1 or 0 depending on direction of scroll. This was being coerced into a uint8_t, discarding the sign. The problem was masked by the coercion happening when the value is put into the mouse descriptor's int8_t.

It's not clear to me why process_wheel_user uses int16_t for its h and v parameters, when the mouse_report values are only int8_t, and dir only takes values of 1, -1 and 0. Perhaps int8_t is more appropriate for the dir variable? Maybe even opt_encoder_handler? I matched the dir type to the function which produces it, and changed nothing that didn't strictly need to change.

[fauxpark]

opt_encoder_handler() should ideally return an int8_t to save space (and really we should not be using int etc. for clarity), otherwise looks good.

[drashna]

opt_encoder_handler() should ideally return an int8_t to save space (and really we should not be using int etc. for clarity), otherwise looks good.

That's probably because a lot of the code came from arduino originally. And it's something that I've been meaning to clean up.