Add support for ETC1 textures

This is implemented in TP but does not work for AE yet-
I am trying to sort it out.
Merge pull request #181 from nazgee/for-nicolas-ETC1-TP

Add support for ETC1 textures
Updated classpath.
Added clear method to Library.
Add CascadingAlphaModifier which will apply the alpha value for entity itself
and all its descendant children entities.
Merge remote-tracking branch 'official/GLES2-AnchorCenter' into AnchorCenter