gl: rename platform cocoa to cgl

gl: rename platform cocoa to cgl

Before:
GST_GL_PLATFORM=cocoa GST_GL_WINDOW=cocoa
gst-launch-1.0 videotestsrc ! glimagesink

After:
GST_GL_PLATFORM=cgl GST_GL_WINDOW=cocoa
gst-launch-1.0 videotestsrc ! glimagesink

but still pass --enable-cocoa to configure script
because currently it can only be used with cocoa API.

We could later have cgl/gstglcontext_cgl.h that manages
a CGLContextObj directly and cocoa/gstglcontext_cocoa.h
would just wrap it.
So that it could be used with other Apple's window APIs.

https://bugzilla.gnome.org/show_bug.cgi?id=729245