Add const to draw functions taking in v2* arrays

[RandyGaul]

Quite annoying to cast around in user code.

e.g.

CF_API void CF_CALL cf_draw_polyline(CF_V2* points, int count, float thickness, bool loop)

->

CF_API void CF_CALL cf_draw_polyline(const CF_V2* points, int count, float thickness, bool loop)