Measure SpriteFont in perf.log

Uses FT_Load_Char in our SpriteFont code as suggested in https://github.com/OpenRA/OpenRA/issues/3436. Compare also http://www.freetype.org/freetype2/docs/tutorial/step1.html. Adds performance measurements to the pre cache methods. Spoiler: this does not change anything in that regard. Makes use of the face creation simplification from the https://github.com/Robmaister/SharpFont/#cross-platform-freetype-bindings-for-net README.