cmake: Always use multiproc build in MSVC

Multicore processors have become the norm, and the minimal rebuild
setting only works on i386 anyway.
cmake: Make Freetype mandatory for non-headless builds

OpenClonk fails to start without Freetype support, because it cannot
load required fonts.
C4Object: Don't do any drawing calcs when headless

There's no point in calculating render positions or audio source
locations when we're not rendering any audio or graphics. Skip it when
headless to save CPU cycles.
CStdFont: Encapsulate iLineHgt
C4FontLoader: minimize footprint when headless

There's no need to do font loading and layouting when we're not going to
render it anyway.
Dynamic particles: skip calculations and rendering when headless

Particles are not required for synchronicity in network play, so we can
skip all calculations concerning them in headless mode.