vala: update to 0.50.3.

Vala 0.50.3
===========
* Various improvements and bug fixes:
  - codegen:
    + Use CCodeInvalidExpression instead of place holders
    + Don't leak memory of already assigned out-parameter on error [#1123]
    + Don't leak memory on internal value comparison of property setter
    + Fix assignment of casted struct value to property [#1126]
  - vala:
    + Report an error if gio-2.0 is missing for DBus support
    + Add missing TraverseVisitor.visit_addressof_expression()
    + value_type of PointerIndirection expressions must not be owned [#1118]
    + SliceExpression need to return heap-allocated or unowned references [#1120]
    + Accept "unowned var" as type for foreach variable declaration [#152]
    + Ownership transfer of inline-allocated array is not allowed [#931]
  - tests: Use Automake窶冱 parallel test driver to speed up running tests [#1094]
  - testrunner: A lot of simplifications

* Bindings:
  - gio-2.0: Fix DBusSubtreeIntrospectFunc binding
  - gstreamer-1.0: Fix direction of GLib.Value typed parameters [#1014]
  - gstreamer: Update from 1.19.0+ git master
  - gtk4: Use correct cheader_include for wayland/x11 gdk backend [#1112]
  - gtk4: Don't rename binding for gtk_css_provider_load_from_data [#1117]
  - gtk4: Update to 4.0.1
  - webkit2gtk-4.0: Update to 2.30.3

(wiz)