Enable the rendering of all items when rendering a screenshot
Unify two almost identical methods into one
Move inline private method implementation in cpp file
Use two independent constraint solvers - vertical and horizontal

This will be used to improve performance by making each solver cache
some data.
Reorder constraint solver API calls according to documentation
Reuse the LP data structure when solving anchor layout constraints.

Speeds up things quite a bit, as no memory allocations are needed and a
lot of things are preinitialized.