This PR implements the viewport overlay for Weight Paint mode in GPv3.
In Weight Paint mode the stroke points are colored depending on their
weights in the active vertex group.
Pull Request: https://projects.blender.org/blender/blender/pulls/118273
Now that all relevant code is C++, the indirection from the C struct
`GPUVertBuf` to the C++ `blender::gpu::VertBuf` class just adds
complexity and necessitates a wrapper API, making more cleanups like
use of RAII or other C++ types more difficult.
This commit replaces the C wrapper structs with direct use of the
vertex and index buffer base classes. In C++ we can choose which parts
of a class are private, so we don't risk exposing too many
implementation details here.
Pull Request: https://projects.blender.org/blender/blender/pulls/119825
Overall the transition to C++ in the draw module is awkwardly half
complete, but moving more code to a C++ namespace makes cleaning up
this code in other ways much easier, and the next C++ cleanup steps
are clear anyway.
