Represent BufferSource and ArrayBufferView as ordinary IDL typedefs over
their underlying union types, instead of special casing in the IDL
generator. This allows the union conversion/return machinery handle
these types consistently with other typedefs, which removes buffer
specific paths from the IDL generator.
This necessitates changing the WebIDL::BufferSource and
WebIDL::ArrayBufferView classes as views over these variants. This
replaces the old GC backed BufferableObject wrapper structure and
provide convenience helpers to determine things such as the byte length,
byte offset, backing buffer, and typed-array APIs.
Represent WebIDL C++ types with a single CppType model that tracks
nullability, optional presence, and contained storage.
GC-like values now use GC::Ref/GC::Ptr directly, while containers choose
"plain", "Root", or "Conservative" container types depending on what
they contain. For example, sequence<Element> becomes a RootVector of
GC::Ref values, while sequence<SomeDictionary> becomes a
ConservativeVector only when the dictionary contains GC-like values.
This moves the generated bindings away from wrapping GC values in
GC::Root by default.
This has broad fallout as the types passed to interfaces for GC
objects changes almost fully across the board.
This IDL change is needed for webaudio's WaveShaperNode, where a
null BufferSource for a curve attribute results in a zero-length buffer.
WebGL also has a nullable BufferSource arg in bufferData(...). But
there, a null data/srcData value returns GL_INVALID_VALUE.
