Failed ptrcalls: instead of panic, print error + return default

[Bromeon]

For inbound (GDScript->Rust) calls done via ptrcall convention, the library used to panic on parameter conversions.

While ptrcalls typically don't fail as their type is already verified at the call site (e.g. in GDScript), there are still some edge cases:

• user panics
• enum values outside the valid range
• narrowing integer conversions (not yet tested)

Except for user panics, the new implementation no longer causes a panic, and instead uses Result internally. This should be beneficial in panic=abort setups or for platforms which don't always have panics/exceptions enabled (e.g. Wasm).

Thanks a lot to @Houtamelo for bringing this to our attention! 👍

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Implementation details

Ptrcalls look like this on the FFI level:

unsafe extern "C" fn ptrcall_fn(
    method_data: *mut std::ffi::c_void,
    instance_ptr: sys::GDExtensionClassInstancePtr,
    args_ptr: *const sys::GDExtensionConstTypePtr,
    ret: sys::GDExtensionTypePtr,
);

Godot pre-initializes the ret value: if it is not assigned by the extension binding (godot-rust), the default value for that type is used: null pointer for objects, 0 for numbers, identity for transforms, etc. So failing ptrcalls will not cause UB -- this PR also adds tests for this behavior.