[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.
[Linux] fixed horizontal font alignment

kAlignTop and kAlignBottom should center text horizontally
Merge pull request #1931 from Weeds/feature_linux-load-ttf

fixed #1688: [Linux] fixed loading truetype fonts from resource directory.
Merge pull request #1932 from Weeds/feature_linux-font-alignment

fixed #1689: [Linux] fixed horizontal font alignment.
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