Add `getRGBVector3i()` to every point type with RGB

Currently only some of the point types with RGB have methods to access their RGB data as an Eigen vector.

This commit moves definitions of `getRGBVector3i()` and `getRGBVector4i()` inside `PCL_ADD_RGB` macro so that every point type with RGB field gets these functions.

It is not a part of this pull request, but I would propose to rename `getRGBVector4i()` into `getRGBAVector4i()` to make it clear that it produces a vector where last value is alpha channel. What do you think?