docs: Finish documenting graphene_rect_t
rect: Allow changing up modifiers

We should return the modified argument, so that we can chain up callers.
This is especially useful in higher order languages.
docs: Add section for graphene_matrix_t
docs: Document versioning macros
docs: Document the GRAPHENE_RECT_INIT macro
docs: Document the vector length macros
docs: Add gtk-doc overrides for SIMD types

We don't want to parse graphene-config.h, and gtk-doc would get it
mostly wrong anyway. We use simple overrides to the effect that the
graphene_simd4f_t and graphene_simd4x4f_t become opaque struct types.