Currently a descriptor set is represented with a structure
whose fields are the various descriptors that the shader is
going to use, each of them qualified with its type. This model
doesn't match very well what happens with D3D12 and with Vulkan
and descriptor indexing and mutable descriptors, where many
descriptor of different types can overlap, and only at shader
runtime it is decided which of them must be used.
Therefore with this patch a descriptor is represented as a
structure whose fields carry no intrinsic typing information,
but are reinterpreted according to the shader behavior on
each access. For the moment there is just one field, but more
will be added to account for fancier descriptor types and
to workaround Metal limitations.
When completed, this design will be similar in spirit to what
the Metal shader converter does, with little technical
differences.
This choice has a couple of drawbacks:
1. shader debugging with Xcode is less comfortable, because
the shader doesn't carry static descriptor typing
information and Xcode is therefore less able to provide
useful context;
2. it requires tier 2 argument buffer support and macOS
version >= 13, which includes guarantees on the runtime
layout for descriptors.
Supporting descriptor indexing and mutable descriptors is
considered to be worthy of those drawbacks, though, is
effectively required by many applications and appears to be
the first goal we should aim for. If needed, in the future
we might also add support for older hardware, at least for
shaders that do not make use of advanced features.
Like we do for GLSL; there's no reason to abort compilation here. Note
that this also avoids leaking "gen->buffer" and "gen->string_buffers" on
the error path.
