Add alpha property to movit.opacity filter.
Add some movit.opacity presets for fading.
Merge pull request #37 from jliljebl/luma-lift-gain-gamma

Add lumaliftgaingamma filter to plusgpl
These locale functions are defined by glibc, not Linux.
Implement LC_NUMERIC handling for non-glibc and non-Darwin OS.
Change LC_NUMERIC support to use GLIBC instead of linux define.
Piggy-back on Movit's GL.h library.

This works around the issue that Movit might want to change from GLEW
to something else in the (very near) future; it's maybe not very pretty,
but it works pretty well in practice.