Entities:
   spline
   block
   xref
   insert

Make binary font format that reads faster, has malloc info at front, and
is closer to X, and avoids transforming same point twice.

Selection grid.

Maybe: (optimizations)

Compile dimensions into entity lists, add an entity pointer to dimension.  All that logic/math is too intensive.

Turn multiple XDrawLine's into XDrawLines.

Store angles as radians, since that's what sin/cos/tan want.  Store degree angles alongside to accelerate display, since X wants that.

