This adds generic attribute rendering support for meshes for Eevee and
Workbench. Each attribute is stored inside of the `MeshBufferList` as a
separate VBO, with a maximum of `GPU_MAX_ATTR` VBOs for consistency with
the GPU shader compilation code.
Since `DRW_MeshCDMask` is not general enough, attribute requests are
stored in new `DRW_AttributeRequest` structures inside of a convenient
`DRW_MeshAttributes` structure. The latter is used in a similar manner
as `DRW_MeshCDMask`, with the `MeshBatchCache` keeping track of needed,
used, and used-over-time attributes. Again, `GPU_MAX_ATTR` is used in
`DRW_MeshAttributes` to prevent too many attributes being used.
To ensure thread-safety when updating the used attributes list, a mutex
is added to the Mesh runtime. This mutex will also be used in the future
for other things when other part of the rendre pre-processing are multi-threaded.
`GPU_BATCH_VBO_MAX_LEN` was increased to 16 in order to accommodate for
this design.
Since `CD_PROP_COLOR` are a valid attribute type, sculpt vertex colors
are now handled using this system to avoid to complicate things. In the
future regular vertex colors will also use this. From this change, bit
operations for DRW_MeshCDMask are now using uint32_t (to match the
representation now used by the compiler).
Users should be warned that when using the color or vector output of the Attribute
node with a scalar attribute, it will be read as `vec4(v, 0, 0, 1)` (i.e. a red-ish color),
instead of the usual conversion to `vec4(v, v, v, 1)` (i.e. a white-ish color) which we are
all accustomed to. This is a limitation of the current shading system, and will be
resolved during or after the EEVEE rewrite.
Ref T85075