fix Droid Sans Fallback not registered to crengine

Mupdf 1.5 ships with "DroidSansFallback.ttc" instead of the "DroidSansFallback.ttf". The new ttc file has two fonts, the original "Droid Sans Fallback H" and a vertical variant "Droid Sans Fallback V". This patch fixes registering ttc files and makes "Droid Sans Fallback H" the fallback font for crengine.