gl(base)memory, videoinfo, glallocationparams: Pass self by const in getters and readonly scenarios

Safe excerpt from !1004 (closed), complements gstreamer!730 (merged)

This MR improves const-correctness - in particular for Rust - around GLFilter function signatures while those bindings are being added. Treewide replacement has also been performed for VideoInfo and (GL)AllocationParams, the latter following a similar change in the gstreamer repo.

Unfortunately we cannot update gst_gl_allocation_params_copy with const which again passes itself to a copy Func that we cannot update for reasons described in linked MRs. gst_gl_allocation_params_copy_data has also not been updated because gst_gl_allocation_params_init takes some of its members as non-const.

There's a single conflict when backporting this to 1.18 around gst_video_converter_new_with_pool, can we still get this backported? As mentioned in gstreamer!730 (merged) there should be no breaking API changes (anymore).