Use the same attribute conversion class for PBVH and regular
mesh attribute extraction. This makes the GPU formats we use
for each attribute type more explicit and centralizes the conversions
from the attribute types to the GPU types. It's also a bit more aligned
to how we could use implicit sharing for GPU vertex buffer data.
Unfortunately it isn't possible to use this same code for curves
and point clouds because they use textures for their evaluated data,
and 3-wide vectors (e.g. `float3`) aren't supported on GPUs with our
current texture abstraction. For generic attributes, the long term
approach will probably be to use an SSBO instead.
Pull Request: https://projects.blender.org/blender/blender/pulls/114340
