Excess default constructor for brush_props

[hatcat]

Paper does not declare default brush_props constructor, yet implementation provides one.

[mikebmcl]

The paper and the implementation are almost in accord. That said, the paper is broken. It effectively has two default ctors. The paper has a ctor with default values for every parameter. It also supplies default values for its member data and those values diverge in regard to the default value of the io2d::filter member data value. So we should eliminate the ctor that has params that all have default values and should choose between filter::good, filter::bilinear (the two that they differed on), or some other filter enumerator.

[mikebmcl]

I slightly misspoke. The implementation is what is broken. Fixing the implementation and sticking with the paper's filter::good value.