allocator: Make GstAllocationParams function parameter const
For Rust we'd really like G-IR annotations to be on point to save us from overriding mutability of various parameters, and at the same time enforcing the same rules here in C. All uses of
plugins-base have been consistently checked and converted to
const, for which the same has to be done here in
GstAllocator::alloc and a few other functions.
Can we get this backported to 1.18? I think @slomo said changing signatures like this is "fine", but it may cause code to emit
incompatible-pointer-types warnings (as it does currently with the
gst-plugins-* repos), eventually as errors if enabled. It'd be nice for us to import these changes to