Merge pull request #931 from hwhw/master

alpha-blitting widget and a few fixes
fix run out of memory when receiving large files from Calibre
also paint widget if any widget underneath is dirty
ignore koreader output file for clang target
Merge pull request #932 from chrox/master

fix run out of memory when receiving large files from Calibre
fix #933
Merge pull request #934 from chrox/fix_933

fix #933
fix the nil tags data bug
Merge pull request #936 from WS64/master

fix the nil tags data bug
trace package loading process in koreader
refatoring: lazy loading of packages
move platform dependent files into 'platform' directory
Added acquisition relation types , fixes issue #940
Merge pull request #945 from evgsh/opds

opds / Added acquisition relation types , fixes issue #940
Merge pull request #944 from chrox/lazy_loading

refactoring: lazy loading of unused modules
adapt to new freetype text rendering without background

now that our text rendering doesn't render a solid color background
anymore, the widget abstractions need to take this into account.