display: replace glib deprecated api
win32: #include GL/glext.h on windows

also add some GL default variables if some GL types are not defined
display: constify GstGLFuncs *gl
feature: add gstreamer license
feature: try and catch more GL extension functions

if we don't find the extension in core GL, try the extension version anyway
upload: provide nicer debugging
windwo_win32: port to new API
memory: fix GSIZE printf format on win32
window_win32_wgl: implement wglGetProcAddress for extension functions
build: add explicit dependency on GModule
opengl: fix plugins not being registered when they should be
overlay: tentative port to new GstGLFuncs API