Part of #118145.
Instead of storing a separately allocated array for each BVH node
for the temporary "color buffer" colors meant for mixing during a
stroke, just store an array the size of the whole mesh. Though this
is wasteful in terms of memory usage, plenty of other brushes store
mesh-sized arrays already, and it should make more sense as BVH
nodes get smaller too. After this commit, the BVH tree has no
specific code for color attributes anymore.
d282b1735e