Replace bespoke text rendering functionality with SDL2-ttf

[no-lex]

libSDL2 has text rendering code in one of its libraries which can handle more sophisticated text rendering than the tessfont/rendertext combo currently used.

libsdl2-ttf is readily available and much more usable than the undocumented code in the engine at present, and the engine is already wed to the libSDL2 ecosystem for its sound and texture handling.