Some SDL-related compiling tweaks

* -lSDL2_mixer is already added to SDL_LDFLAGS by default, unless NOMIXER=1 is set
* -DSDLMAIN should also be added to OPTS by default for MINGW=1 builds, unless NOSDLMAIN=1 is set

NiGHTS hotfix

Fixes the following issues relating to playing as NiGHTS Super Sonic that apparently popped up between 2.1.14 and next (mostly due to the changes to SRB2's trig stuff it seems):
* Super Sonic drifts to the side at some angles around an axis, and is unable to go directly upwards or downwards as a result
* Drilling to the side when on the ground causes the drill sound to constantly restart
* CEZS's start not actually being lined up properly with the first axis means the player is not able to go backwards along the track (because the player is not actually aligned with the track properly, preventing you from touching the attached line transfer)
* trying to hug some walls such as the tall wall before the library section of CEZS allows Super Sonic to go through them

These fixes needs proper testing before this branch can be merged in, in case they accidentally break other things as a result or something.

See merge request !71

(no actual SLOC changes)

If player is at wrong distance from axis, correct x/y position in a similar fashion to how momentum is set for homing attacks. Don't use trig here for most cases since that just re-introduces the side drift issue and friends all over again.

This fixes the CEZS issue where the player start is not actually on the track, preventing the player from going backwards until they hit a wall.