This avoids OpenCL inlining heavy volume interpolation code once for every
data type, which could cause a performance regression when we add a float4
data type in the next commit.
Ref D2057
This patch adds a new Vertex Color node. The node also returns the alpha
of the vertex color layer as an output.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5767
