build: Require GLib ≥ 2.43.2

We need g_set_object().
area: Use g_set_object()
basicmaterial: Use g_set_object()
directionallight: Add properties

And accessors, where necessary.
mesh: Add properties
shadermaterial: Add properties

And accessors, where needed.
Remove duplicate declaration
Remove duplicate declaration
build: Enable introspection

Still a ton of errors, and apparently g-ir-scanner has some issue with
the Graphene types.
Add some Graphene types annotations

The introspection scanner does not automatically associate a CamelCase
type to the C type, even if the corresponding introspection data clearly
matches the two. This means that we'll have to manually annotate all
Graphene types uses.

This is a simple commit that shows how.
build: Remove unnecessary scanner arguments
Merge pull request #3 from ebassi/set-object

Use g_set_object()
Merge pull request #4 from ebassi/introspection

Add support for introspection