Feat/prelight additions

[mwls-sean]

Added percent brightness support to the SHOW.PRE_LIGHTS param. Existing integer values retain their level of brightness.

[CLAassistant]

All committers have signed the CLA.

[mwls-sean]

Compiler didn't like comparisons of variables to float values. Maybe there is a utility that already provides this?

[ntamas]

Judging from the code in AP_Param.cpp, the raw storage format of a parameter in the EEPROM depends on its type. This PR changes the type of the SHOW_PRE_LIGHTS parameter from AP_Int8 to AP_Float so I think that previous values of this parameter saved into EEPROM would not be preserved if we merge this PR and a user upgrades the firmware on a flight controller from an earlier version of the Skybrush firmware to this one.

I'm putting this PR on hold for the time being. If there's considerable interest from the community to merge this feature, drop a message here. In that case, some kind of an upgrade mechanism will have to be provided that re-writes the type of the parameter in the EEPROM from AP_Int8 to AP_Float.

[ntamas]

Proposal:

• Let's keep the type of the parameter as AP_Int8.
• Values less than 5 remain interpreted as it is now (i.e. bit shifts on the RGB components).
• Values greater than or equal to 5 but less than or equal to 100 are interpreted as percentages so you can still achieve almost arbitrary scaling of the RGB values.
• Values greater than 100 are interpreted as 100.

[seankovacs]

Yea, sorry. I already did this for my own builds. Guess I forgot to push up.

[ntamas]

Superseded by 36a3a2c in the CMCopter-4.5 branch. (But thanks for your contribution anyway!).