[Linux] fixed loading truetype fonts from resource directory

We should use CCFileUtils to get the full path to font resources.
Additionally the input font is converted to lowercase for checking if it is
a ttf file. So we can also load .TTF or .tTf files.
Merge pull request #1931 from Weeds/feature_linux-load-ttf

fixed #1688: [Linux] fixed loading truetype fonts from resource directory.