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Merge pull request #201 from hmaddocks/fix_crash_in_PolycodeFrame_handleEvent

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Added virtual to UIWindow destructor

Fixes issue #198. See http://stackoverflow.com/questions/461203/when-to-use-virtual-destructors
Fixed screen root entity being adjusted by default, fixed image alpha premultiplication
Merge branch 'master' of https://github.com/ivansafrin/Polycode
Merge pull request #206 from CIB/cleanup

Added virtual to UIWindow destructor
Added a flag to renderer to treat normal blending as premultiplied, made the screen entity size default to 0 instead of 1
Merge branch 'master' of https://github.com/ivansafrin/Polycode
Fixed collision only entity not setting rotation correctly
Fixed point collision examples
Quick fix for the compilation of the IDE on Windows Debug.

Problem is that both physfs and libpng compiles zlib statically.
The compiler now ignores multiple definitions and executes just fine.
It still displays warnings instead of errors, in case one day we make libpng / physfs compile zlib dynamically
Merge branch 'fix-IDE-Windows' of github.com:aserre/Polycode into fix-IDE-Windows