[OverlayRenderGL] Fix corrupt pixels in graphical subs

On Pi you can see a corrupt line of pixels on bottom and right edges of subtitle bounding box.
This is due to using a subregion of a texture (due to the power of two rounding) and not ensuring
the texture coordinates are inside the valid pixels.

The easy solution is to use NPOT when available (always on GLES) and then the clamping works
(as well as saving memory).
updated addon helpers

in particular, we now handle buildsystems which output several add-ons
properly. real life case; screensavers.rsxs
[WIN32] added a xbmc-config.cmake.win32 for windows
[WIN32] added xbmc_find_path to find paths recursively
[WIN32] removed prepare_addon_environment(). this is done in the prepare-env step now.
move to addon helpers. fits better and avoids duplication for win32
[cmake] add lto support again
[cmake] fix tgz package and set zip as default cpack_generator
Merge pull request #3126 from cptspiff/fix_multiaddon

updated addon helpers
GUILabelControl optimization
Merge pull request #3114 from stupid-boy/GUILabelControl

GUILabelControl optimization
Merge pull request #3116 from popcornmix/gfxsubcorrupt

[OverlayRenderGL] Fix corrupt pixels in graphical subs
[airplay] - always force internal dvdplayer for airplaying, fixes non working airplay if user has configured external player
[osx] - sync version to alpha7