gl: Fix compilation with GLES2
examples/clutter: Include gst-plugins-base CFLAGS and LIBS to fix uninstalled compilation
examples: Stop using deprecated GLib thread API
glfilter: Use uint64 for the OpenGL context property

ulong/long properties are deprecated and should not be used anymore
for portability reasons.
Merge pull request #1 from sdroege/wip-platform

Thanks